Numerous thyroid issues are investigated with thyroid function testing. A thyroid test assesses the levels of many hormones, including triiodothyronine (T3), thyroxine (T4), and thyroid stimulating hormone (TSH) (T3). Various antibodies associated with thyroid tissue are tested as part of further thyroid procedures.
The pituitary gland in the brain creates TSH. The thyroid gland, located in the front of the neck, receives TSH from the blood. TSH then affects the rate of T4 and T3 release into the blood.
Blood samples can be used to check the levels of TSH, T3, and T4. If your TSH is abnormal, your doctor will likely test TSH first, followed by T3 and T4. To identify autoimmune thyroid disease, tests can also be done to assess antibodies.
Different Thyroid Blood Test Types?
A series of blood tests known as thyroid function tests are performed to gauge how well your thyroid gland is functioning. They provide information about other linked disorders and aid in detecting an overactive or underactive thyroid gland. Thyroid blood tests can be of three primary types:
- TSH test: The thyroid gland does not secrete the stimulating thyroid hormone (TSH). It is produced in the brain’s pituitary gland. When thyroid hormone levels fall in your body, the pituitary gland releases more TSH so that the thyroid gland can adjust by releasing more thyroid hormones. The pituitary gland produces less TSH when the thyroid hormone level is high. As a result, excessively high or low TSH readings might indicate whether your thyroid is functioning correctly.
- T4 test: Determines the amount of the hormone T4 in the blood (thyroxine).
- T3 test: Evaluates the blood levels of T3, or triiodothyronine, the other primary thyroid hormone.
These hormones may be measured using blood tests, commonly used but not appropriate in all circumstances. The following are tests to gauge thyroid function:
Using a blood sample to measure TSH is the most accurate method for determining thyroid function. The fluctuation of TSH can serve as an “early warning system,” occurring before the body’s actual amount of thyroid hormones reaches an unhealthy level.
A high TSH level implies insufficient thyroid hormone production by the thyroid gland (primary hypothyroidism). When the TSH level is low, the opposite circumstance often means the thyroid overproduces thyroid hormone (hyperthyroidism).
Rarely, a low TSH might be brought on by a pituitary disorder that prevents the gland from producing enough TSH to activate the thyroid (secondary hypothyroidism).
An increased TSH and a low FT4 or FTI level point to primary hypothyroidism brought on by thyroid gland illness. Low TSH and low FT4 or FTI levels signify hypothyroidism brought on by a pituitary gland issue. In those with hyperthyroidism, the TSH is low while the FT4 or FTI is increased.
T3 tests are frequently helpful for diagnosing hyperthyroidism or determining its severity. A patient with hyperthyroidism will have a higher-than-normal T3 level. Some people with low TSH merely have increased T3 levels; their FT4 or FTI levels are normal. Since T3 testing is the last test to become abnormal, it is rarely beneficial in hypothyroid patients.
The primary type of thyroid hormone found in the blood is T4. Total T4 can alter when binding proteins differ since it measures both bound and free hormones (see above). A Free T4 measures unbound substances that can enter and impact bodily tissues. When combined with a TSH test, tests measuring free T4—either a free T4 (FT4) or free T4 index (FTI)—better indicate the thyroid gland’s health.
Although measuring free T3 is technically possible, it is frequently unreliable and rarely helpful.
Reverse T3 is a physically highly similar protein to T3, but since the iodine atoms are arranged differently, it is not physiologically active. The body naturally produces some reverse T3 but quickly breaks down. Measuring reverse T3 is not clinically relevant in healthy, non-hospitalized individuals since it does not assist in determining whether hypothyroidism is present.
Thyroid issues can create a number of symptoms that might have an impact on your life. Thus, it is essential to get the test done if you are experiencing any such issues. However, the average thyroid test cost in India ranges from ₹500 to ₹750.