What to expect during a thyroid function test

Have you been advised to get a thyroid function test? Here is all you need to know about the tests.

Shanaya, a 30 year old working mother was lately saying that she felt extremely tired, had put on a few kilograms and did not feel like eating. So she consulted her local doctor who, referred her to an endocrinologist and asked her to get her thyroid functions evaluated. Here is everything you need to know about the thyroid function test.

What is thyroid function?

Thyroid tests are usually ordered when you experience one or more of the following symptoms.

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  • Tremors,
  • Increased heart rate,
  • Dry skin,
  • Hair loss,
  • Weight gain or loss,
  • Difficulty in sleeping,
  • Anxiety,
  • Abnormal periods and,
  • Intolerance to cold.

The thyroid function tests are performed to check the functioning of the thyroid gland , which is regulated by the hypothalamus ( a gland in your brain which releases TRH) which in turn sends a signal to the pituitary gland ( another gland in your brain) to release TSH (Thyroid Stimulating Hormone). This TSH stimulates the thyroid gland to produce thyroid hormones (T3 and T4) with the help of iodine, a protein and an enzyme called TPO. The thyroid hormones once released into the blood bind to proteins (thyroid binding globulin), so the active form of the hormone is the unbound form or free form.

TSH (Thyroid Stimulating Hormone)

Done to diagnose thyroid disorders (hyperthyroidism or hypothyroidism) or to monitor treatment. This test measures the amount of TSH in blood. Blood is drawn from the veins in the arm after taking septic precautions.

Prerequisites: Overnight fasting is required. Drugs such as steroids, oral contraceptive pills interfere with the test, so it is always better to inform your doctor in case you are taking any of these medications.

These days ultrasensitive third generation tests are used to measure the TSH in your blood.

Interpreting the results: Normal TSH levels are -- 0.5- 4.5 mIU/L. But according to the new guidelines of American Association of clinical Endocrinologists the normal values are 0.3- 3.04 mIU/ L.

T4 or Thyroxine

Your thyroid gland produces a hormone called thyroxine or T4. This hormone helps in growth and metabolism. It circulates in your body bound to a protein, so the free form (or the unbound form) is the active form. For this reason, free T4 (FT4) levels are preferred over total T4.This test measures the amount of thyroxine in blood. Usually this test is ordered if your doctor sees that you have either abnormal TSH levels, would like to test you for a thyroid disorder or as a part of monitoring treatment. For this test blood is drawn from the veins in the arm after taking septic precautions.

Prerequisites: Overnight fasting is required, drugs such as steroids, birth control pills, androgens may interfere with the test, therefore it is best you consult your doctor before taking the test.

Interpreting the results: Normal FT4 levels are between 4.5- 11.2 mcg/ dL. High T4 levels indicate hyperthyroidism and low T4 indicates hypothyroidism.

T3 or Triiodothyronine

It helps in growth and development along with T4. T3 is ten times stronger than T4 in its action. It circulates in the body bound to protein, the unbound form is the active form. The free T3 levels are measured when you have abnormal TSH value or if your doctor orders an entire panel of TSH (free T3 and free T4 together). Blood is drawn from the vein in your arm after taking septic precautions

Prerequisites: Overnight fasting is required for the test. Inform your doctor if you are taking steroids or oral contraceptive pills as they may interfere with the test value.

Interpreting the results: Normal free T3 values are- 2.3-4.2 pg/ ml. A low T3 may indicate hypothyroidism and high T3 hyperthyroidism. Further a high TSH, low T3 and low T4 indicates hypothyroidism, while a low TSH, high T3 and high T4 indicate hyperthyroidism.

Cost of the test: The thyroid function test costs approximately Rupees 500 to 800, depending upon the lab.

FNAC or fine needle aspiration cytology

If you have a visible swelling on your throat, you may be asked to undergo the FNAC test. Also known as fine needle aspiration cytology, where a needle is used to take a small sample from the area (where the swelling is located). This is then studied for abnormalities under the microscope.

Prerequisites: No preparation is required for this test. The only exception here would be if you have been advised to have an ultrasound of the neck. In such cases, you should have the ultrasound before an FNAC, as the aspiration may give erroneous results on ultrasonograph.

Cost of the test: This test costs approximately Rupees 500, and is commonly performed at most pathological labs.

Thyroid scan

This is a test which scans the thyroid gland to see how well it works and to detect any lumps in the gland. This test uses radionuclides which emit gamma rays. You may be asked to swallow radionuclides or may be given an injection. In case you take the pill, you have to wait for around 5 hours for the test to be done. If you are injected the test is done after half an hour. The rays are picked up by a gamma camera and converted into an image by the computer. Functional part of the gland will appear 'hot' and the part of the gland which is inactive will appear 'cold' as in the case of thyroid cancer. The test is usually safe as it uses almost the same amount of radiation as an X-ray.

Precautions: It is to be avoided during pregnancy and breast feeding.

Prerequisites: You will be asked to fast overnight before the tests and will be asked to stop your thyroid medicines a week before. Also you will be asked to avoid eating shellfish as they contain iodine and may interfere with the results.

Cost of the test: The price varies from one lab to another. Considering it is a specialized test, it is available at modern nuclear medicine centers only.

Antithyroid antibodies

The immune system produces some antibodies which destroy the thyroid tissue causing clinical signs and symptoms of thyroid disease. Antithyroid antibodies are tested to establish an autoimmune disease which might be the reason for thyroid dysfunction. Done usually to follow up a case of goitre or when a person has been diagnosed with hyperthyroidism or hypothyroidism, or is suffering from an autoimmune disease and has developed symptoms related to thyroid disease. To test for the presence of antithyroid antibodies, the following antibodies are tested:

1) Thyroid peroxidase antibody (TPOAb)/ Antimicrosomal antibody: Thyroid peroxidase is an enzyme required for the synthesis of T3 and T4.

Sample collection: Sample is collected from a vein in your arm after overnight fasting.

Interpreting the results: Normal values are less than 9 IU/ ml, a positive antibody titre indicates Hashimoto's disease, Myxedema, Graves' disease, other autoimmune diseases.

2) Thyroglobin antibody (TgAB)

Sample collection: Sample is collected from the vein after overnight fasting.

Interpreting the results: Normal values are less than 116 IU/ ml.Positive titres are seen in Hashimotos's disease, other autoimmune diseases.

3) Thyroid stimulating immunoglobulin (TSI)/ Thyroid stimulating hormone receptor ( TSHR Ab)

Sample collection: Sample is collected from the vein after overnight fasting.

Interpreting the results: The levels of these antibodies are found elevated in Graves disease. The testing of antibodies is done only at a few selected diagnostic centers.

Image source: Getty Images

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