Thyroid Function Tests and Hypothyroidism

Thyroid Function Tests and Hypothyroidism

Thyroid Function Tests and Hypothyroidism - Thyroid-Stimulating Hormone (TSH) TSH Levels Test & Thyroid Antibodies Test » Thyroid Function Tests And Their ImportanceThyroid hormones affect virtually every cell and tissue in the body. Their main function is to regulate metabolism in addition to promoting proper brain development, controlling the heart and nervous system, maintaining accurate body temperature, controlling menstrual cycles, cholesterol levels, body weight, etc. Thus, they are one of the most important hormones regulating body functions and fluctuations in their levels can wreck havoc in the patients’ health.

Thyroid hormones are produced by the thyroid which is a small butterfly shaped gland that is located in the front of the neck, just below the larynx. There are two types of thyroid hormones, T3 or Triiodothyronine and T4 or Tetraiodthyronine, also called thyroxine.

These hormones are made up of iodine in combination with the amino acid tyrosine. T4 is produced almost exclusively in the thyroid whereas T3 is produced mainly in the liver and kidney by removing one iodine atom from T4 with the help of the enzyme thyroid peroxidase (TPO).

T3 is about four times more active than T4 and the circulating levels of T4 and T3 are roughly 80% and 20% respectively. The thyroid hormones circulate in two forms – bound and free. In the bound form, the hormones are linked to a thyroid protein called thyroglobulin or thyroid binding globulin (TBG). This is the inactive form of the hormones as they cannot enter tissues and exert their effect. The free form is the active form of the hormones.

Thyroid Stimulating Hormone (TSH) is an important hormone secreted by the pituitary that regulates the production of thyroid hormones. The TSH and the thyroid hormones work in a well-coordinated feedback loop system where the levels of TSH drop when circulating levels of T4 and T3 rise and increases when the levels of T4 and T3 declines. The TSH is in turn regulated by another hormone called TSH Releasing/Regulating Hormone (TRH) secreted by the hypothalamus.

Thyroid function tests are ordered when a patient presents with symptoms of hyperthyroidism like drastic weight loss, rapid heart beat, anxiety, tremors, insomnia, intolerance to heat, etc. or hypothyroidism like sudden weight gain, lethargy, intolerance to cold, menstrual irregularities, constipation, fatigue, etc.

There are a number of tests to assess thyroid function and are resorted to depending on the severity of symptoms. The major tests and their interpretations are as follows:


This is an extremely sensitive test and is the first test that is usually performed to reach a tentative diagnosis. Under normal circumstances, an elevated level of TSH suggests hypothyroidism whereas a decreased level is indicative of hyperthyroidism. In the former case, the pituitary secrets excess TSH to stimulate the thyroid gland to produce more hormone as the circulating levels are less and in the latter case, the TSH production is shut off by the pituitary following an increase in the levels of T4 and T3.

Sometimes, low levels of TSH maybe due to a problem with the pituitary gland rather than the thyroid that prevents it from releasing enough TSH into circulation. This leads to a condition called secondary hypothyroidism as, in the absence of TSH, sufficient T4 and T3 are not secreted. Similarly, hyperthyroidism may result due to benign tumors in the pituitary that causes it to release excess TSH into circulation thereby increasing T3 and T4 levels but this is an extremely rare condition.

Normal range: 0.3 to 4 mIU/L (may vary from laboratory to laboratory)

T4 and free T4 tests

Total T4 test measures the circulating levels of both bound and free T4. Free T4 (FT4) test measures the circulating levels of unbound T4 only. Under normal circumstances, elevated levels of T4/ FT4 suggest hyperthyroidism and low levels suggest hypothyroidism.

The levels of total T4 maybe misleading in certain cases when erroneous results are obtained due to fluctuations in the level of protein-bound T4. Normal levels of FT4 and too high or low levels of total T4 point to the fact that the problem lies in the levels of binding protein and not with the thyroid.

Thyroid Function Tests and Hypothyroidism - Thyroid-Stimulating Hormone (TSH) TSH Levels Test & Thyroid Antibodies Test » Thyroid Function Tests And Their Importance

The levels of binding protein may increase during pregnancy or with the use of oral contraceptives and may decrease following severe illness or use of steroids. The total T4 levels will in these cases be high and low respectively suggesting hyperthyroidism in the first case and hypothyroidism in the second. Thus FT4 measurement gives a more accurate diagnosis of a patient’s condition as compared to total T4 test.

Normal range: Total T4: 4.5 to 12.6 microgram per deciliter (may vary from laboratory to laboratory)

Free T4: 0.7 to 1.8 nanograms per deciliter (may vary from laboratory to laboratory)

T3 and free T3 tests

Total T3 and free T3 (FT3) tests are used mainly to detect hyperthyroidism and are not very helpful in hypothyroidism diagnosis. The levels of FT3 may be elevated even when FT4 levels are within normal range in some cases of hyperthyroidism. Thus FT3 helps in early detection of hyperthyroidism. In the case of hypothyroidism, the levels of FT3 are normally within the correct range even when FT4 levels drop and remain so till hypothyroidism is severe.

In the case of T3 also, as with T4, free hormone tests are more accurate than total hormone tests.

Normal range: Total T3: 60 to 181 nanogram per deciliter (may vary from laboratory to laboratory)

Free T3: 0.2 to 0.5 ng/dL (may vary from laboratory to laboratory)

Thyroid Antibody Tests

Antibodies are substances that are produced by the body in response to pathogens or antigens. Even though this is a useful protective mechanism, sometimes, antibodies are produced against the body’s own cells, proteins, antigens, etc.

Anti thyroid antibodies are produced against the two thyroid proteins, thyroglobulin and thyroid peroxidase, in Hashimoto’s disease which is the most common cause of hypothyroidism.

Thyroid Function Tests and Hypothyroidism - Thyroid-Stimulating Hormone (TSH) TSH Levels Test & Thyroid Antibodies Test » Thyroid Function Tests And Their Importance

Anti TG antibodies attack thyroglobulin and anti-TPO antibodies attack thyroperoxidase which lead to low levels of circulating T3 and T4. Presence of anti-TG and/or anti TPO antibodies can easily confirm the diagnosis of Hashimoto’s disease.Presence of anti-thyroid antibodies in a patient with hyperthyroidism can be indicative of autoimmune thyroid disease.

Radioactive Iodine Uptake (RAIU)

The thyroid takes up iodine from the bloodstream in order to synthesize the thyroid hormones. By ingesting radioactive iodine, this uptake can be measured. The measurement is done using a gamma probe which is positioned over the thyroid and takes less than ten minutes to perform.

For the test, the patient is made to drink a small amount of liquid containing radioactive iodine, or it is given in the form of a capsule. The iodine uptake is measured after about four to six hours and then again after twenty four hours. If the RAIU is very high, it suggests that the patient has hyperthyroidism caused due to Garves’ disease or enlargement of the thyroid. This test is not used to detect hypothyroidism.

RAIU can be used to differentiate between hyperthyroidism and thyroiditis. In the former case, the uptake of iodine is high whereas in the latter case, the uptake is low. Thyroiditis is caused due to the inflammation of the thyroid which causes thyroid hormones to leak into the bloodstream in large amounts and mimic the symptoms of hyperthyroidism.

While thyroiditis is a self-limiting disease that returns to normalcy once the excess thyroid hormones are eliminated from the body (typically six to eight weeks), hyperthyroidism requires long term treatment. Thus, a correct diagnosis is crucial for proper treatment and follow up.

Thyroid Scan

Thyroid scan also requires the patient to ingest radioactive iodine or radioactive Technetium. These may alternatively be injected into a vein which is the more preferred method. The thyroid is scanned thirty minutes after injection of the radioactive substance using a gamma camera.

This scan also detects the amount of iodine uptake by the thyroid like the RAIU test, but in addition shows the presence of nodules in the thyroid. The nodules may be benign or cancerous, but this can’t be distinguished by the scan although it helps to differentiate between ‘hot’ nodules and ‘cold’ nodules.

Hot nodules are ones that absorb more radioactive iodine than normal thyroid tissue while cold nodules do not absorb much iodine. Hot nodules are usually not cancerous but cold nodules have a high probability of being so.

Fine Needle Aspiration (FNA)

This is a method of obtaining cells from the thyroid (usually the nodule) to assess their benign or malignant nature. A fine needle is injected into the nodule and cells are aspirated. These cells are then stained on a microscopic slide and studied by a pathologist. This is usually the only test required to ascertain the malignancy of a thyroid nodule.

Thyroid Ultrasound

This test is similar to other ultrasound tests and uses high frequency sound waves to create an image of the thyroid. This test can help distinguish between solid nodules and fluid filled ones. Although, like thyroid scan, it cannot differentiate between benign and malignant nodules, it can help in the correct location of nodules for fine needle aspiration surgery.

TRH Test

Though not very common, this test is used to detect hyperthyroidism and hypothyroidism in the early stages. TRH (TSH Releasing Hormone) injections can cause increases in TSH levels. If a person has a high amount of thyroid hormones (hyperthyroidism), TSH will not increase following a TSH injection. Likewise, if the TSH levels increase drastically following TRH injections, then it is indicative of hypothyroidism.

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