Normal Thyroid Hormone Levels – Endocrine Surgery

What is thyroid hormone?

Thyroid hormone is made by the thyroid gland, a butterfly-shaped endocrine gland normally located in the lower front of the neck. Thyroid hormone is released into the blood where it is carried to all the tissues in the body. It helps the body use energy, stay warm and keeps the brain, heart, muscles, and other organs working as they should.

Thyroid hormone exists in two main forms: thyroxine (T4) and triiodothyronine (T3). T4 is the primary form of thyroid hormone circulating in the blood (about 95%). To exert its effects, T4 is converted to T3 by the removal of an iodine atom; this occurs mainly in the liver and in certain tissues where T3 acts, such as in the brain. T3 normally accounts for about 5% of thyroid hormone circulating in the blood.

Most thyroid hormone in the blood is bound by protein, while only a small fraction is “free” to enter tissues and have a biologic effect. Thyroid tests may measure total (protein bound and free) or free hormone levels.

Production of thyroid hormone by the thyroid gland is controlled by the pituitary, another endocrine gland in the brain. The pituitary releases Thyroid Stimulating Hormone (abbreviated TSH) into the blood to stimulate the thyroid to make more thyroid hormone. The amount of TSH that the pituitary sends into the bloodstream depends on the amount of thyroid hormone in the body. Like a thermostat, if the pituitary senses low thyroid hormone, then it produces more TSH to tell the thyroid gland to produce more. Once the T4 in the bloodstream goes above a certain level, the pituitary’s production of TSH is shut off. In this way, the pituitary senses and controls thyroid gland production of thyroid hormone. Endocrinologists use a combination of thyroid hormone and TSH testing to understand thyroid hormone levels in the body.

What is a TSH test?

Thyroid tests
Blood tests to measure low or high TSH levels are readily available and widely used. Not all thyroid tests are useful in all situations.

TSH Test
The best way to initially test thyroid function is to measure the TSH (Thyroid Stimulating Hormone) level in a blood sample. Changes in TSH can serve as an “early warning system” — often occurring before the actual level of thyroid hormones in the body becomes too high or too low.

A high TSH level indicates that the thyroid gland is not making enough thyroid hormone (primary hypothyroidism). On the other hand, a low TSH level usually indicates that the thyroid is producing too much thyroid hormone (hyperthyroidism). Occasionally, a low TSH may result from an abnormality in the pituitary gland, which prevents it from making enough TSH to stimulate the thyroid (central hypothyroidism). In most healthy individuals, a normal TSH value means that the thyroid is functioning properly.

What is a T4 test?

T4 Tests
A Total T4 test measures the bound and free thyroxine (T4) hormone in the blood. A Free T4 measures what is not bound and able to freely enter and affect the body tissues.

What does it mean if T4 levels are abnormal?
Importantly, Total T4 levels are affected by medications and medical conditions that change thyroid hormone binding proteins. Estrogen, oral contraceptive pills, pregnancy, liver disease, and hepatitis C virus infection are common causes of increased thyroid hormone binding proteins and will result in a high Total T4. Testosterone or androgens and anabolic steroids are common causes of decreased thyroid hormone binding proteins and will result in a low Total T4.

In some circumstances, like pregnancy, a person may have normal thyroid function but Total T4 levels outside of the normal reference range. Tests measuring free T4 – either a free T4 (FT4) or free T4 index (FTI) – may more accurately reflect how the thyroid gland is functioning in these circumstances. An endocrinologist can determine when thyroid disease is present in the context of abnormal thyroid binding proteins.

What is a T3 test?

T3 Tests
T3 tests measure triiodothyronine (T3) levels in the blood. A Total T3 test measures the bound and free fractions of triiodothyronine. Hyperthyroid patients typically have an elevated Total T3 level. T3 tests can be used to support a diagnosis of hyperthyroidism and can determine the severity.

In some thyroid diseases, the proportions of T3 and T4 in the blood change and can provide diagnostic information. A pattern of increased T3 vs T4 is characteristic of Graves’ disease. On the other hand, medications like steroids and amiodarone, and severe illness can decrease the amount of thyroid hormone the body converts from T4 to T3 (active form) resulting in a lower proportion of T3.

T3 levels fall late during hypothyroidism and therefore are not routinely used to evaluate patients with underactive or surgically absent thyroid glands.

Measurement of Free T3 is possible but is often not reliable and therefore may not be helpful.

What is a normal thyroid (hormone) level?

To assess thyroid hormone status for low or high TSH levels we use TSH and FT4 tests. The normal value for a laboratory test is determined by measuring the hormone in a large population of healthy individuals and finding the normal reference range. Normal ranges for thyroid tests may vary slightly among different laboratories, and typical ranges for common tests are given below.

TSH normal values are 0.5 to 5.0 mIU/L. Pregnancy, a history of thyroid cancer, history of pituitary gland disease, and older age are some situations when TSH is optimally maintained in a different range as guided by an endocrinologist.

FT4 normal values are 0.7 to 1.9ng/dL. Individuals taking medications that modify thyroid hormone metabolism and those with a history of thyroid cancer or pituitary disease may be optimally managed with a different normal FT4 range.

Total T4 and Total T3 levels measure bound and free thyroid hormone in the blood. These levels are influenced by many factors that affect protein levels in the body, including medications, sex hormones, and liver disease.
A normal Total T4 level in adults ranges from 5.0 to 12.0μg/dL.
A normal Total T3 level in adults ranges from 80-220 ng/dL.

Free T3 assays are often unreliable and not routinely used to assess thyroid function.

What does it mean if my thyroid levels are abnormal?

Patterns of thyroid tests associated with thyroid disease

Primary Hypothyroidism
A high TSH and low thyroid hormone level (e. g. low FT4) can indicate primary hypothyroidism. Primary hypothyroidism occurs when the thyroid gland makes too little thyroid hormone. Symptoms of hypothyroidism can include feeling cold, constipation, weight gain, slowed thinking, and decreased energy.
Causes of primary hypothyroidism include:

• Autoimmune thyroid disease, including Hashimoto’s thyroiditis
• Thyroid gland dysfunction due to a medication (e.g. amiodarone, tyrosine kinase inhibitors, or cancer immunotherapy)
• Removal of all or part of the thyroid gland
• Radiation injury to the thyroid (e.g. external beam radiation, radioactive iodine ablation treatment)
• Excess treatment with anti-thyroid medications (e.g. methimazole, propylthiouracil)

Early or mild hypothyroidism may present as a persistently high TSH and a normal FT4 hormone level. This pattern is called subclinical hypothyroidism and your doctor may recommend treatment. Over time, untreated subclinical hypothyroidism can contribute to heart disease.

It is important to remember that normal TSH levels in older individuals (ages 70 and above) are higher than the normal ranges for younger individuals.

Primary Hyperthyroidism
A low TSH and a high thyroid hormone level (e.g. high FT4) can indicate primary hyperthyroidism. Primary hyperthyroidism occurs when the thyroid gland makes or releases too much thyroid hormone. Symptoms of hyperthyroidism can include tremors, palpitations, restlessness, feeling too warm, frequent bowel movements, disrupted sleep, and unintentional weight loss.
Causes of primary hyperthyroidism include:

• Graves’ disease
• Toxic or autonomously functioning thyroid nodule
• Multinodular goiter
• Thyroid inflammation (called thyroiditis) early in the course of disease
• Thyroid gland dysfunction due to a medication (e.g. amiodarone or cancer immunotherapy)
• Excess thyroid hormone therapy

Early or mild hyperthyroidism may present as a persistently low TSH and a normal FT4 hormone level. This pattern is called subclinical hyperthyroidism and your doctor may recommend treatment. Over time, untreated subclinical hyperthyroidism can worsen osteoporosis and contribute to abnormal heart rhythms.

Thyroiditis
Thyroid inflammation, also called thyroiditis, causes injury to the thyroid gland and release of thyroid hormone. Individuals with thyroiditis usually have a brief period of hyperthyroidism (low TSH and high FT4 or Total T4) followed by development of hypothyroidism (high TSH and low FT4 or Total T4) or resolution.

Some forms of thyroiditis are transient, like post-partum thyroiditis or thyroiditis following an infection, and often resolve on their own without need for medication.

Other forms of thyroiditis, like thyroiditis resulting from cancer immunotherapy, interferon alpha, or tyrosine kinase inhibitors, usually result in permanent hypothyroidism and require long term treatment with thyroid hormone replacement.

Your endocrinologist will monitor your thyroid tests during thyroiditis and can help determine if you need short- and long-term medications to balance your thyroid function and control any symptoms.

Central Hypothyroidism
A low TSH and a low FT4 may indicate pituitary disease. Detection of central hypothyroidism should prompt your doctor to check for problems in other pituitary hormones, an underlying cause, and you may need imaging tests to look at the pituitary gland.

Central hypothyroidism is treated with thyroid hormone replacement. Importantly, adequacy of thyroid replacement in central hyperthyroidism is assessed with FT4 and Total T4 tests not TSH as in primary hyperthyroidism, and deficiency in stress hormone cortisol should be assessed before starting thyroid treatment to prevent an adrenal crisis. 

Causes of central hypothyroidism include pituitary gland disease, such as a pituitary mass or tumor, history of pituitary surgery or radiation, pituitary inflammation (called hypophysitis) resulting from autoimmune disease or cancer immunotherapy, and infiltrative diseases.

Rare causes of abnormal thyroid function
Thyroid hormone resistance
Iodine induced hyperthyroidism
TSH-secreting tumor (TSH-oma)
Germ cell tumors
Trophoblastic disease
Infiltrative diseases, such as systemic scleroderma, hemochromatosis, or amyloidosis.

When abnormal thyroid function tests are not due to thyroid disease

While blood tests to measure thyroid hormones and thyroid stimulating hormone (TSH) are widely available, it is important to remember that not all tests are useful in all circumstances and many factors including medications, supplements, and non-thyroid medical conditions can affect thyroid test results. An endocrinologist can help you make sense of thyroid test results when there is a discrepancy between your results and how you feel. A good first step is often to repeat the test and ensure there are no medications that might interfere with the test results. Below are some common reasons for mismatch between thyroid tests and thyroid disease.

Non-thyroidal illness
Significant illness, such as an infection, cancer, heart failure, or kidney disease, or recent recovery from an illness can cause transient changes in the TSH. Fasting or starvation can also cause a low TSH. An endocrinologist can help to interpret changes in thyroid function tests in these circumstances to distinguish non-thyroid illness from true thyroid dysfunction.

Test interference
Biotin, a common supplement for hair and nail growth, interferes with many thyroid function tests and can lead to inaccurate results. Endocrinologists recommend stopping biotin supplements for 3 days before having a blood test for thyroid function.

Individuals who have exposure to mice, like laboratory researchers and veterinarians, may develop antibodies against mouse proteins in their blood. These antibodies cross react with reagents in multiple thyroid function tests and cause unpredictable results. A specialized assay can accurately measure thyroid hormone levels and TSH in this circumstance.

I don’t feel well, but my thyroid tests are normal

Thyroid blood tests are generally accurate and reliable. If you do not have low or high TSH levels and your thyroid tests are normal, your symptoms may not be related to thyroid disease, and you may want to seek additional evaluation with your primary care physician.

How is hypothyroidism treated?

What is thyroid medication?

Thyroid Hormone Treatment
Levothyroxine is the standard of care in thyroid hormone replacement therapy and treatment of hypothyroidism. Levothyroxine (also called LT4) is equivalent to the T4 form of naturally occurring thyroid hormone and is available in generic and brand name forms.

How do I take levothyroxine?
To optimize absorption of your thyroid medication, it should be taken with water at a regular time each day. Multiple medications and supplements decrease absorption of thyroid hormone and should be taken 3–4 hours apart, including calcium and iron supplements, proton pump inhibitors, soy, and multivitamins with minerals. Because of the way levothyroxine is metabolized by the body, your doctor may ask you to take an extra pill or skip a pill on some days of the week. This helps us to fine tune your medication dose for your body and should be guided by an endocrinologist.

For patients with celiac disease (autoimmune disease against gluten) or gluten sensitivity, a gluten free formulation of levothyroxine is available. Some individuals may have genetic variant that affects how the body converts T4 to T3 and these individuals may benefit from the addition of a small dose of triiodothyronine.

Liothyronine is replacement T3 (triiodothyronine) thyroid hormone. This medication has a short half-life and is taken twice per day or in combination with levothyroxine. Liothyronine alone is not used for treatment of hypothyroidism long term.

Other formulations of thyroid hormone replacement include natural or desiccated thyroid hormone extracts from animal sources. Natural or desiccated thyroid extract preparations have greater variability in the dose of thyroid hormone between batches and imbalanced ratios if T4 vs T3. Natural or animal sources of thyroid hormone typically contain 75% T4 and 25% T3, compared to the normal human balance of 95% T4 and 5% T3. Treatment with a correct balance of T4 and T3 is important to replicate normal thyroid function and prevent adverse effects of excess T3, including osteoporosis, heart problems, and mood and sleep disturbance. An endocrinologist can evaluate symptoms and thyroid tests to help balance thyroid hormone medications.

How do I know if my thyroid dose is correct?

Monitoring thyroid levels on medication
Correct dosing of thyroid hormone is usually assessed using the same tests for diagnosis of thyroid disease, including TSH and FT4. Thyroid tests are typically checked every 4-6 weeks initially and then every 6 to 12 months once stable. In special circumstances, such as pregnancy, a history of thyroid cancer, central hypothyroidism, amiodarone therapy, or use of combination T4 and T3 thyroid hormone replacement, your endocrinologist may check different thyroid tests. Additionally, your endocrinologist will evaluate for symptoms of hyperthyroidism and hypothyroidism and perform a physical exam.

Women who are pregnant and women who may become pregnant should only be treated with levothyroxine (T4). Only T4 efficiently crosses the placenta to provide thyroid hormone to the developing fetus. Thyroid hormone is critical in early pregnancy for brain development. Normal ranges for thyroid tests in pregnancy are different and change by trimester. Women with thyroid disease in pregnancy or who are considering pregnancy should be under the care of an endocrinologist to guide therapy.

Individuals with a history of thyroid cancer, even if only a portion of the thyroid was removed, also have different target ranges for TSH and FT4 tests. Thyroid hormone replacement in these individuals is closely tied to ongoing thyroid cancer surveillance, monitoring of thyroid cancer tumor markers, and dynamic assessment of recurrence risk. These patients are optimally managed by a multidisciplinary team including an endocrinologist and endocrine surgeon.

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TSH test Information | Mount Sinai

Thyrotropin; Thyroid stimulating hormone; Hypothyroidism – TSH; Hyperthyroidism – TSH; Goiter – TSH

A TSH test measures the amount of thyroid stimulating hormone (TSH) in your blood. TSH is produced by the pituitary gland. It prompts the thyroid gland to make and release thyroid hormones into the blood.

Endocrine glands release hormones (chemical messengers) into the bloodstream to be transported to various organs and tissues throughout the body. For instance, the pancreas secretes insulin, which allows the body to regulate levels of sugar in the blood. The thyroid gets instructions from the pituitary to secrete hormones which determine the rate of metabolism in the body (the more hormone in the bloodstream, the faster the chemical activity; the less hormone, the slower the activity).

The pituitary secretes thyroid stimulating hormone (TSH), which stimulates the thyroid gland to secrete hormones that affect body metabolism.

How the Test is Performed

A blood sample is needed. Other thyroid tests that may be done at the same time include:

• T3 test (free or total)
• T4 test (free or total)
• Anti-TPO (antibodies against thyroid peroxidase)

How to Prepare for the Test

There is no preparation needed for this test. Ask your health care provider about any medicines you are taking that may affect the test results. DO NOT stop taking any medicines without first asking your provider.

Medicines you may need to stop for a short time include:

• Amiodarone
• Dopamine
• Lithium
• Potassium iodide
• Prednisone or other glucocorticoid medicines

The vitamin biotin (B7) can affect the results of the TSH test. If you take biotin, talk to your provider before you have any thyroid function tests.

How the Test will Feel

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or a slight bruise. This soon goes away.

Why the Test is Performed

Your provider will order this test if you have symptoms or signs of an overactive or underactive thyroid gland. It is also used to monitor treatment of these conditions.

Your provider may also check your TSH level if you are planning to get pregnant.

Normal Results

Normal values range from 0. 5 to 5 microunits per milliliter (µU/mL).

TSH values can vary during the day. It is best to have the test early in the morning. Experts do not fully agree on what the upper number should be when diagnosing thyroid disorders. Some labs will use a higher normal range limit for older people (even as high as 7 µU/mL).

Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your provider about the meaning of your specific test results.

If you are being treated for a thyroid disorder, your TSH level will likely be kept between 0.5 and 4.0 µU/mL, except when:

• A pituitary disorder is the cause of the thyroid problem. A low TSH may be expected.
• You have a history of certain types of thyroid cancer. A TSH value below the normal range may be best to prevent the thyroid cancer from coming back.
• A woman is pregnant. The normal range for TSH is different for women who are pregnant. Your provider may suggest that you take thyroid hormone, even if your TSH is in the normal range.

What Abnormal Results Mean

A higher-than-normal TSH level is most often due to an underactive thyroid gland (hypothyroidism). There are many causes of this problem.

A lower-than-normal level may be due to an overactive thyroid gland, which can be caused by:

• Graves disease
• Toxic nodular goiter or multinodular goiter
• Too much iodine in the body (due to receiving iodine contrast used during imaging tests, such as CT scan)
• Taking too much thyroid hormone medicine or prescribed natural or over-the-counter supplements that contain thyroid hormone

Use of certain medicines may also cause a lower-than-normal TSH level. These include glucocorticoids/steroids, dopamine, certain chemotherapy drugs, and opioid painkillers such as morphine.

Risks

There is little risk involved with having your blood taken.Veins and arteries vary in size from one person to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight, but may include:

• Excessive bleeding
• Fainting or feeling lightheaded
• Multiple punctures to locate veins
• Hematoma (blood buildup under the skin)
• Infection (a slight risk any time the skin is broken)

Guber HA, Oprea M, Rusell YX. Evaluation of endocrine function. In: McPherson RA, Pincus MR, eds. Henry’s Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 25.

Jonklaas J, Cooper DS. Thyroid. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 213.

Salvatore D, Cohen R, Kopp PA, Larsen PR. Thyroid pathophysiology and diagnostic evaluation. In: Melmed S, Auchus RJ, Goldfine AB, Koenig RJ, Rosen CJ, eds. Williams Textbook of Endocrinology. 14th ed. Philadelphia, PA: Elsevier; 2020:chap 11.

Weiss RE, Refetoff S. Thyroid function testing. In: Jameson JL, De Groot LJ, de Kretser DM, et al, eds. Endocrinology: Adult and Pediatric. 7th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 78.

Last reviewed on: 1/9/2022

Reviewed by: Robert Hurd, MD, Professor of Endocrinology and Health Care Ethics, Xavier University, Cincinnati, OH. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

Medical Testing for Thyroid Diseases

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Published:
03/11/2017

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Updated:

05/14/2022

• Thyroid
• Stress

The thyroid gland is involved in the regulation of metabolism. The gland produces hormones that are responsible for the process of using energy. Almost all cells and organs are under the influence of thyroid hormones. An imbalance of thyroid hormones leads to disruption of most systems and organs.

Thyroid hormones

The thyroid gland uses iodine, obtained mainly from food, to produce hormones. The most important hormones are thyroxine (T ₄ ) and triiodothyronine (T ₃ ), which account for 99.9% and 0.1% of thyroid hormones in the blood, respectively. When released from the gland, the hormone T ₄ turns into the hormone T ₃ – an active hormone that affects cell metabolism. At the peripheral level, it is the hormone T ₃ has major physiological effects.

The activity of the thyroid gland is under the control of the pituitary gland. The pituitary gland influences the activity of the gland through thyroid-stimulating hormone (TSH). When the thyroid gland reduces the production of its hormones, the pituitary gland produces additional amounts of TSH in an attempt to stimulate the organ. In cases where the production of hormones is increased, the pituitary gland suppresses the production of TSH, and stimulation of the gland occurs to a lesser extent. Thanks to such a system of regulation of the work of the body, called the “negative feedback loop”, the level of thyroid hormones is kept in a certain range of values.

Malfunction of the organ can occur at any level: the thyroid gland, pituitary gland or peripheral tissues, leading to the development of hyper- and hypothyroid states.

When thyroid hormones are produced in excess, hyperthyroidism develops, and when there is a deficiency, hypothyroidism develops. In 95% of cases, the pathology is present in the organ itself, the remaining 5% are diseases of the hypothalamic-pituitary system.

Medical diagnosis of hyperthyroidism

Hyperthyroidism is associated with a condition where the body suffers from a large amount of T9 hormones.0019 3 and T ₄ . This condition is also called thyrotoxicosis. The most common cause of hyperthyroidism is an autoimmune disease called Graves’ disease. In this disease, the body produces special antibodies to the tissue of the gland, which cause the thyroid gland to produce hormones in excess.

Hyperthyroidism can be caused by toxic nodular or multinodular goiters, which are growths of glandular tissue capable of producing hormones. In addition, excessive iodine intake and uncontrolled intake of iodine-containing drugs also lead to hyperthyroid conditions.

Symptoms of hyperthyroidism:

• fatigue and muscle weakness;
• hand tremor;
• mood swings;
• nervousness or restlessness;
• rapid heartbeat;
• dry skin;
• sleep problems;
• weight loss;
• increased frequency of defecation.

Medical diagnosis of hypothyroidism

Hypothyroidism is associated with an underactive thyroid gland. There are two main causes of the disease.

The first is the result of previous or current inflammation in the thyroid tissue. The inflammatory process leads to the defeat of a large percentage of the body’s cells, which become unable to produce enough hormones. The most common cause of thyroid failure is autoimmune thyroiditis, also called Hashimoto’s thyroiditis.

The second main cause falls into the category of “treatments”. Treatment of many thyroid conditions requires surgical removal of part or all of the organ. If the total mass of thyroid cells left in the body is not enough to meet the needs of the body, then hypothyroidism develops. These surgeries are performed for thyroid cancer.

Symptoms of hypothyroidism:

• fatigue;
• hypersensitivity to cold;
• constipation;
• dry skin;
• weight gain;
• plump face;
• hoarseness of voice;
• muscle weakness and pain;
• elevated blood cholesterol;
• pain, stiffness or swelling in the joints;
• irregular menstrual periods that are difficult;
• fine hair;
• slow pulse;
• depression;
• weakened memory.

Depending on the severity of the deficiency of thyroid hormones and the length of time during which the body was deprived of their required amount, the list of symptoms and their severity will vary. Some patients with hypothyroidism have no symptoms of the disease, or have such a mild expression that they go unnoticed. It is not uncommon for symptoms of the disease, such as fatigue or weight gain, to be mistakenly associated with age-related changes, and therefore hypothyroidism remains undiagnosed for a long time.

Diagnosis of thyroid diseases

To diagnose the functions of the thyroid gland, laboratory and instrumental diagnostic methods are used. The doctor will order one or more blood tests to confirm the diagnosis. Screening laboratory tests include: a blood test for thyroid-stimulating hormone (TSH), T ₄ , T ₃ and tests for antibodies to the thyroid gland.

Thyroid blood tests

1) TSH test

TSH is a hormone produced in the pituitary gland that tells the thyroid gland how much T ₄ and T ₃ need to be synthesized.

A high TSH level often means that there is organ failure. The thyroid gland does not produce enough of the hormone, but the pituitary gland continues to synthesize and secrete TSH.

When hypothyroidism occurs, there is usually a lack of T9 hormones in the blood0019 3 and T ₄ which can be directly measured. However, at the onset of the disease, the level of these hormones may still be normal. Thus, the main tool for detecting hypothyroidism is to measure the level of thyroid-stimulating hormone.

Low TSH levels indicate that the thyroid gland is overactive and produces too much hormone, so the pituitary gland inhibits TSH synthesis.

If your TSH test results are not normal, you will need at least one more test to help find the cause of the problem.

2) Test T ₄

A high level of T ₄ in the blood may indicate the presence of hyperthyroidism, a low level may indicate hypothyroidism.

In some cases, high or low T levels ₄ are not associated with thyroid problems. During pregnancy or while taking oral contraceptives, the level of thyroid hormones will be higher. Taking corticosteroids helps to reduce the concentration of T ₄ . Medications and some conditions change the amount of blood proteins that “bind” and transport the hormone.

Bound T ₄ stored in the blood until needed. “Free” T ₄ is not associated with carrier proteins and is available to target tissues. Therefore, many doctors prefer to measure free T ₄ .

3) Test T ₃

In cases where there is a suspicion of hyperthyroidism, the value of T ₄ corresponds to the normal concentration range, it is necessary to determine the level of T ₃ . Sometimes T ₄ is normal, but T ₃ is high, so it is important to measure both hormone levels at the same time. Like the hormone T ₄ , it is possible to determine two fractions of T ₃ : total and free. Free T ₃ has the main biological effect, therefore it is of greater interest in terms of diagnosing hyperthyroid conditions.

4) Thyroid antibody tests

Determination of thyroid antibodies can help diagnose autoimmune thyroid disorders such as Graves’ disease, the most common cause of hyperthyroidism, and Hashimoto’s disease, the most common cause of hypothyroidism. Thyroid antibodies are formed when the immune system attacks the thyroid gland by mistake. There are two main types of antibodies that are determined in the laboratory: antibodies to thyroperoxidase and antibodies to thyroglobulin.

Treatment

An endocrinologist deals with the treatment of thyroid diseases. The prognosis of the disease is quite successful, especially when organ dysfunction is detected in the early stages. The results of treatment largely depend on the causes of the disease.

• Thyroid
• Stress

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*Review of the 2015 European Thyroid Association guidelines on diagnosis and treatment of endogenous subclinical hyperthyroidism

Introduction 9029 5

In the normal functioning of the female reproductive system, the work of the thyroid gland ( thyroid gland) plays one of the key roles. That is why an obstetrician-gynecologist, in the presence of any disorders in this area, excludes this widespread endocrine pathology, which can be the cause of a number of pathological conditions: sexual development disorders, amenorrhea (primary or secondary), anovulatory cycles, infertility, miscarriage, and requires specific treatment, which will be determined by the nature of hormonal status disorders: hypothyroidism or thyrotoxicosis. And often only laboratory diagnostics can give a final answer regarding the diagnosis, since clinical manifestations are not always strictly specific (for example, menstrual irregularities can occur in both hypothyroidism and thyrotoxicosis).

One of the most common and widely used tests to identify patients with thyroid disease is the determination of thyroid stimulating hormone (TSH). This test at the screening stage of identifying the presence or absence of functional disorders of the thyroid gland is recognized in the world as the most optimal and sufficient. Evaluation of the TSH level deviation depending on age, gender, presence or absence of pregnancy, trimester of pregnancy allows us to identify those patients in whom we can assume the presence of thyrotoxicosis (subclinical or overt) or hypothyroidism (central) if a reduced level of TSH is detected. In the case of an increase in the level of TSH, the presence of hypothyroidism (subclinical or overt) is assumed.

To make a final diagnosis and choose treatment tactics, the levels of peripheral thyroid hormones are determined, taking into account the presence of antibodies that help to differentially diagnose thyrotoxicosis or central hypothyroidism, subclinical or manifest form of the disease, with a reduced level of TSH, and with an elevated level of TSH, make a diagnosis of subclinical or overt hypothyroidism.

Today, one of the problematic topics in endocrinology is the variants of thyroid dysfunction, which belong to the category of subclinical forms: subclinical hyperthyroidism and hypothyroidism. In these situations, a specialist always faces a number of questions to which there are no definitive answers: is the clinical picture associated with thyroid pathology and is it necessary to treat, if so, how? After all, the levels of peripheral hormones, which are responsible for the development of the clinical picture of the disease, are within the reference values, there is only a deviation in the level of TSH.

This article brings to the attention of obstetricians and gynecologists the recommendations of the European Thyroid Association (ETA) guidelines for the diagnosis and treatment of patients with endogenous subclinical hyperthyroidism (ESH), published in 2015 and resulting from a large work of a special group of specialists in this field .The level of evidence for each recommendation is given in accordance with international standards: quality is expressed in pluses (+ low, ++ moderate, +++ high) and strength (1 – strong, 2 – weak). The full text of this guide can be found on the European Thyroid Journal website.

Endogenous subclinical hyperthyroidism (ESH)

ESH may be the result of Graves’ disease, autonomously functioning thyroid nodule, and multinodular goiter. Diagnosis is based on the detection of a permanently reduced (subnormal) TSH level with a level of free thyroid hormones corresponding to the reference limits.

A large number of prospective cohort studies show that ESH is associated with an increased risk of mortality from coronary heart disease, atrial fibrillation, heart failure, fractures, and increased overall mortality in patients with TSH levels <0.1 mIU/L (2nd degree of ESG). Therefore, despite the absence of randomized prospective trials, there is evidence that ESH treatment is indicated in patients over 65 years of age with grade 2 ESH in order to avoid serious cardiovascular consequences, fractures, and the risk of disease progression to overt hyperthyroidism. Treatment may be considered in patients over 65 years of age with a TSH level of 0.1–0.39mIU/L (grade 1 ESG), since ESG is associated with an increased risk of atrial fibrillation, and may also be warranted in patients under 65 years of age with grade 2 ESG due to the risk of progression, especially in the presence of symptoms and /or underlying risk factors or comorbidities. There is no evidence to support the treatment of asymptomatic young patients with grade 1 ESH. Such patients should be observed without treatment due to the low risk of progression to overt hyperthyroidism and weak evidence of adverse health effects.

ESG etiology and prevalence

The most common causes of ESG are Graves’ disease, toxic adenoma and toxic multinodular goiter. While Graves’ disease is the most common cause of ESH in younger patients (<65 years) in an area with normal iodine supply, toxic adenoma and toxic multinodular goiter are relatively more common in iodine-deficient areas in older people (≥65 years) [8].

The prevalence of ESH varies significantly from 0.6 and 16% [8-10], depending on the diagnostic criteria, age and sex of the population, iodine intake. The National Health and Nutrition Examination Survey (NHANES III) assessed the presence of thyroid antibodies, TSH, and free thyroxine (T4f) in people over 12 years of age, which represented the geographic and ethnic distribution of the US population [11]. The prevalence of ESH was 0.7% for TSH cutoff <0.1 mIU/L, and 1.8% for TSH cutoff <0.4 mIU/L. ESH is a relatively common condition in iodine-deficient regions and its prevalence is 15% in the population of people over 70 years of age [12].

Low TSH levels can also be due to iatrogenic (exogenous) causes associated with thyroid hormone treatment: high doses of thyroid hormone in patients with thyroid cancer, in patients treated for hypothyroidism, or for other reasons. This guide only deals with the diagnosis and treatment of ESH.

Diagnosis and determination of the etiology of ESG

Diagnosis of ESG is based solely on laboratory results and not clinical criteria. ESG is determined in the presence of a subnormal TSH level against the background of normal levels of T4f, total triiodothyronine (T3tot) and / or free triiodothyronine (T3f) [2-7].

Modern laboratory testing capabilities can detect extremely low TSH levels of 0.01-0.02 mIU/L. In accordance with this, two categories of ESG can be distinguished [2–7]:

• 1st degree ESG: TSH 0.1–0.39 mIU/l;
• Grade 2 ESG: TSH <0.1 mIU/L.

Serum TSH is the most sensitive test for diagnosing and assessing the severity of ESH [2, 5, 13, 14]. T4f and T3f levels tend to be in the mid-high reference range for subclinical TSH and may help differentiate between ESH and overt hyperthyroidism [3-5]. In hyperthyroidism, serum T3 is often more elevated than T4, due to excessive thyroid production of T3. The total T3 study is often preferred in clinical practice because tests assessing the T3F are less accurate than the T4F study [5, 14]. However, the determination of the T3 level in addition to TSH helps in the early detection of thyroid dysfunction. Its excess is caused by focal or multifocal autonomy of the thyroid gland in patients with goiter living in areas of iodine deficiency [14]. In these patients with an undetectable TSH level and normal levels of T4fw and T3total, the determination of T3fl allows differential diagnosis between ESG and overt T3 thyrotoxicosis [15].

Before making a diagnosis of ESH, qualitative laboratory testing and an adequate interpretation of thyroid hormone levels are necessary, taking into account the possible influence on their levels of a number of factors, including physiological conditions (for example, pregnancy), the presence of intercurrent diseases, drug use and possible artefacts in laboratory testing.

ESH should be differentiated from other causes of low TSH levels of central origin, such as taking drugs that suppress TSH (dopamine, high doses of glucocorticoids, somatostatin analogues, dobutamine, amphetamine, bexarotene, bromocriptine), the presence of mental illness, pseudothyroid dysfunction syndrome in somatic pathology and hypothalamic-pituitary disorders that cause deficiency of thyrotropin-releasing hormone or thyroid-stimulating hormone. The levels of T4fl and T3fl are, as a rule, low normal [4, 6].

Mandatory exclusion of causes that temporarily reduce TSH levels, such as subacute, silent, or postpartum thyroiditis [3–6]. Pregnancy and advanced age can make the diagnosis of ESG difficult. An increase in the concentration of human chorionic gonadotropin can lead to a decrease in TSH in 18% of pregnant women in the early stages, most of whom will have normal concentrations of T4f [16]. In addition, a change in the level of TSH can be observed in elderly patients due to aging and a potential change in the hypothalamus-pituitary-thyroid axis [17, 18].

Iodine intake and/or medications often lead to secondary thyroid changes associated with non-thyroid pathology in the elderly, further complicating the diagnosis of ESH (4).

A number of studies have shown a shift in TSH levels towards higher concentrations in healthy elderly people in regions with normal iodine intake [19]. In contrast, TSH levels may be below the normal range in some healthy centenarians in iodine-deficient areas [20, 21]. A low TSH level, a decrease in serum T3, and an increase in serum reverse T3 suggest an age-related decrease in 5′-deiodinase activity or changes in nutrition in very elderly patients with long-term iodine deficiency [21]. Finally, subnormal serum TSH levels are often observed in dark-skinned people and healthy smokers [4, 6, 22].

Scintigraphy and 24-hour radioactive iodine uptake test differentiate Graves’ disease patients with normally diffuse increased iodine uptake; “warm” or “hot” nodes with toxic multinodular goiter and autonomously functioning thyroid nodes; patients with thyroiditis; patients receiving thyroid hormones and iodine-containing drugs in the absence or reduced absorption of radioactive iodine [2–6, 13].

Measurement of 24-hour urinary iodine excretion can help confirm suspected excessive iodine intake [4, 6].

Color Doppler ultrasound provides information on the size of the thyroid gland, its echogenicity, presence/absence of nodules and vascularization, while fine needle aspiration biopsy will help detect thyroid cancer in multinodular goiter with hypofunctioning nodes [4–6, 13].

Recommendations for the diagnosis of ESH

First level of research: to establish the persistent nature of changes in thyroid hormone levels.

1. We recommend TSH testing as the first level test for diagnosing ESG. If a low level of TSH is detected, it is necessary to investigate thyroid hormones (fT4 and total T3 (or fT3)) [2, 5, 13, 14] (1/+++).
2. TSH should be used to assess the severity of ESH and distinguish gradations: 1st degree (TSH 0.1–0.39 mIU/l), 2nd degree (TSH <0.1 mIU/l) [2, 5, 13, 14] (1/+++).
3. Non-EGS related causes of transient TSH decline, such as drug use, pituitary or hypothalamic insufficiency, mental illness, and euthyroid pathology syndrome [4–7] (1/+), should be ruled out.
4. If TSH levels are low or borderline low and thyroid hormone levels are within or at the upper limit of the normal range, repeat the study after 2-3 months, because ESG is defined as a persistent decrease in TSH levels [2-6] (1/+).

Second level of investigation: to establish the etiology of ESH

5. It is recommended to perform scintigraphy and, if possible, a 24-hour radioactive iodine uptake test to determine the management of patients with nodular goiter and grade 2 ESH [2–6, 13] (1/+).
6. Doppler ultrasound may be informative in patients with ESH and nodular goiter [13] (2/+).
7. Determination of the level of antibodies to the TSH receptor (AT-rTTH) can confirm the autoimmune etiology of induced hyperthyroidism [23] (2/+).

In addition, AR-rTTG will help detect an autoimmune reaction even in nodular goiter, because approximately 17% of patients in iodine-deficient areas with scintigraphic criteria for toxic multinodular goiter may be positive for AT-rTTG [24].

The third level of research: to determine the amount of necessary treatment

Read about the necessary complex of risk assessment associated with ESH, recommendations on the principles of treatment in the next issue.