The thyroid gland is a butterfly-shaped organ situated in the lower front lobe of the neck. This gland works to produce thyroid hormones, which are secreted into the bloodstream and then supplied to all the tissues in the body. The thyroid hormones play a key role in helping the human body utilize energy, maintain body temperature and keep the brain, heart, muscles and other vital organs function normally.

How Does The Thyroid Gland Function?

The main hormone secreted by the thyroid gland is thyroxine (T4) it contains four iodine atoms. To employ its effects, T4 is converted to triiodothyronine (T3) by removing one iodine atom. The amount of T4 made by the thyroid gland is controlled by another hormone, which is produced in the pituitary gland located at the base of the brain, called thyroid-stimulating hormone (TSH). The amount of TSH that the pituitary sends into bloodstream depends on the amount of T4 that pituitary gland sees, if pituitary sees very little T4, then it produces more TSH , which communicates to the thyroid gland to make more T4. Once the level of T4 in the bloodstream goes above a certain limit, the production of TSH is shut off.
Thyroid Function Test

If the thyroid gland doesn’t make enough T3 and T4, a person may experience symptoms like weight gain, lack of energy and depression, this condition is called hypothyroidism. On the other hand, if thyroid gland makes too many hormones, a person may experience weight loss, anxiety, tremors and hypersensitivity, this condition is called hyperthyroidism.

Also Read: Things To Know About Thyroid

Read this article to gain an insight into various thyroid function tests.

Tests To Evaluate Thyroid Functions

TSH Tests

The best method to test thyroid function is to measure the TSH level in the blood. Changes in TSH indicates an early warning that often develops before the actual level of thyroid hormones in the system becomes too high or low. A high TSH level indicates that the thyroid gland is not making adequate thyroid hormones (primary hypothyroidism). When the TSH level is low, it usually indicates that the thyroid gland is making excess thyroid hormone (hyperthyroidism). A low TSH result from an abnormality in the pituitary gland, which prevents it from secreting enough TSH to stimulate the thyroid (secondary hypothyroidism).In healthy individuals, a normal TSH value indicates that the thyroid gland is functioning well.

TSH test is also used to screen new-borns for nay congenital hypothyroidism. Even pregnant women are asked to monitor TSH during the first trimester of pregnancy, as thyroid hormones are specifically needed to ensure the healthy development of the brain and nervous system of the foetus. The thyroid hormones are carried to the baby via the placenta. If the disease is untreated during pregnancy it may lead to premature birth, preeclampsia and low birth. Thus testing TSH hormone levels before and during pregnancy ensure normal values are maintained.

The normal test range of TSH is between 0.4 and 4.0 mlU/L.

If an individual show any signs of hypothyroidism and have a TSH level of above 2.0mlU/L, then a person is at risk of developing hypothyroidism.

Also Read: Hypothyroidism: Check For Underactive Thyroid Gland

T4 Tests

T4 is the main form of thyroid hormone circulating in the blood. Total T4 measures the bound and free hormone and can alter when the binding protein molecules differ. While Free T4 measures what is not bound and able to enter and affect the body tissues. Blood works measuring free T4 –either a free T4 (FT4) or free T4 index (FTI) more precisely reflect how the thyroid gland is functioning when checked with a TSH.

The results of a high TSH and low FT4 or FTI indicates primary hypothyroidism due to disorder in the thyroid gland. A low TSH and low FT4 or FTI indicates hypothyroidism caused due to issue involving the pituitary gland. A low TSH with a high FT4 or FTI is observed in individuals with hyperthyroidism.

T3 Tests

T3 tests are often helpful to diagnosis hyperthyroidism, patients who are hyperthyroid will have an elevated T3 level. While in some person’s with a low TSH, only T3 is high and the FT4 or FTI is normal.

The normal range for T3 is 100-200 ng/dL, abnormally elevated levels mostly indicate a condition called grave’s disease. It is an autoimmune disorder associated with hyperthyroidism.

T3 Resin Uptake Test

 T3 resin uptake (T3RU) is a blood test that determines the binding capacity of a hormone called thyroxin-binding globulin (TBG). If your T3 level is elevated, then your TBG binding capacity would be low.

Abnormally low levels of TBG usually denotes a problem with the kidneys or when the body isn’t getting adequate protein. Whereas abnormally high levels of TBG suggest high levels of estrogen in the body. High estrogen levels may be caused by pregnancy, eating food rich in estrogen, obesity or hormone replacement therapy.

Free T3

Free T3 is also measured, however, it is often not reliable and helpful.

Reverse T3

Reverse T3 is a biologically inactive protein that is structurally related to T3, but the iodine atoms are situated in different locations, which makes it inactive. Reverse T3 is usually produced in the body but degrades rapidly. In healthy adults, reverse T3 does not help to determine hypothyroidism and it is not clinically useful.

Thyroid Antibody Tests

The immune system of the body functions to shield you from foreign invaders such as bacteria and viruses by making antibodies produced by blood cells known as lymphocytes. In patients with a thyroid disorder, lymphocytes react against the thyroid and make antibodies against thyroid cell proteins. Two antibodies - thyroid peroxidase antibody and thyroglobulin antibody. Measuring levels of thyroid antibodies may help to diagnose the cause of the thyroid problem.

 In a patient with hyperthyroidism, a different antibody may be positive is the stimulatory TSH receptor antibody (TSI), this antibody causes the thyroid gland to be overactive in Grave’s Disease. Furthermore, the doctor may also order a thyrotropin receptor antibody tests (TSHR) which detects both stimulating and blocking antibodies. This test may help to determine patient response to the treatment of hyperthyroidism and also assess the risk of passing antibodies to the foetus during pregnancy.


Thyroglobulin (Tg) is a protein made by normal thyroid cells and thyroid cancer cells. It is measured in patients who had surgery for thyroid cancer in order to evaluate the progress of patients after treatment.

Is The Thyroid Test  Done In Fasting State?

No special preparation is needed for TSH blood test, however, it is suggested to get the tests done in the morning, as the TSH levels may fluctuate throughout the day. For follow up tests sample should be given at the same time. Always inform your physician if you are on any medication, as certain drugs can interfere with the test results.


When the blood test results show that your thyroid gland is overactive or underactive, then the doctor may further order a thyroid uptake test or an ultrasound test. These tests will determine for any structural issues with the thyroid gland, gland activity and any tumours that may be causing the issues. Based on these results, the doctor may suggest for a tissue sample from the thyroid gland to check for cancer. Medications are prescribed by the physician to regulate thyroid activity and further tests are done to ensure medication is working.