THYROID FUNCTION TEST

What is the thyroid gland?

The thyroid gland is a butterfly-shaped endocrine gland that is normally located in the lower front of the neck. The thyroid’s job is to make thyroid hormones, which are secreted into the blood and then carried to every tissue in the body. Thyroid hormones help the body use energy, stay warm and keep the brain, heart, muscles, and other organs working as they should.

How does the thyroid gland function?

The major thyroid hormone secreted by the thyroid gland is thyroxine, also called T4 because it contains four iodine atoms. To exert its effects, T4 is converted to triiodothyronine (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. The amount of T4 produced by the thyroid gland is controlled by another hormone, which is made in the pituitary gland located at the base of the brain, called thyroid stimulating hormone (abbreviated TSH). The amount of TSH that the pituitary sends into the bloodstream depends on the amount of T4 that the pituitary sees. If the pituitary sees very little T4, then it produces more TSH to tell the thyroid gland to produce more T4. Once the T4 in the bloodstream goes above a certain level, the pituitary’s production of TSH is shut off. In fact, the thyroid and pituitary act in many ways like a heater and a thermostat. When the heater is off and it becomes cold, the thermostat reads the temperature and turns on the heater. When the heat rises to an appropriate level, the thermostat senses this and turns off the heater.

What is thyroid function test (TFT)

Thyroid function tests are usually done to find out whether the thyroid gland is working properly. This is mainly to diagnose underactive thyroid gland (hypothyroidism) and an overactive thyroid gland (hyperthyroidism).

TSH (Thyroid stimulating hormone) Test: The TSH test is often done first. If the thyroid hormone levels in blood are too low, pituitary gland makes larger amounts of TSH to stimulate production of thyroid hormone and vice versa.

T4 (Thyroxine) Test: A high level of T4 indicates hyperthyroidism. Most of the T4 in body is found to protein and a small percentage is free, together called Total T4. Need to monitor T4 levels if taking thyroid replacement therapy (medication), to check underactive thyroid in new-borns, evaluate conditions like goitre, thyroid nodules and issues with pituitary gland or hypothalamus.

Free T4 Test: A small portion of T4 is not bound to proteins and are called free T4, which are readily available for our body to use. High free T4 above normal range could mean you have an overactive thyroid also seen in Grave’s disease, an autoimmune disorder. Abnormally low free T4 levels may signal hypothyroidism.

T3 (Triiodothyronine)Test: T3 mostly exists as a bound form with proteins. Total T3 refers to the collection of both bound and unbound forms of T3 circulating in the blood. T3 test is most often used to diagnose hyperthyroidism.

Free T3 test: Free T3 are unbound T3 that enters one’s body tissues where its needed. Free T3 is in a small percentage compared to Total T3. However, measuring free T3 is more accurate than measuring Total T3 as it represents the immediately available thyroid hormone which can be used.

 

Thyroid function test Interpretation
TSH	Free T4	Free T3	Condition
Normal	Normal	Normal	None
Low	High	High	Hyperthyroidism
Low	Normal	Normal	Subclinical hyperthyroidism
Low	Normal	High	T3 toxicosis
Low	High	Normal	Thyroiditis T4 ingestion Hyperthyroidism in the elderly or with comorbid illness
Low	Low	Low	Euthyroid sick syndrome Central hypothyroidism
High	Normal	Normal	Subclinical hypothyroidism Recover from euthyroid sick syndrome
High	Low	Low	Primary hypothyroidism
High	High	High	TSH producing pituitary adenoma