What Are Thyroid Function Tests?

Thyroid function tests are laboratory analyses that measure the amount of thyroid hormones in the bloodstream. Several types of thyroid function tests can be used to help diagnose thyroid disease and manage treatment. According to the National Institutes of Health, approximately 6 percent of the U.S. population suffers from some form of thyroid disease.

Role of the Thyroid

• The thyroid gland is located at the front base of the neck. It is part of the endocrine system and is responsible for regulating hormone production, metabolism and protein synthesis throughout the body. It is controlled by the pituitary gland in the brain.

Thyroid Hormones

• Three main hormones are associated with the thyroid: T4, T3 and TSH. T4 (thyroxine) is an inactive hormone produced by the thyroid that is converted into T3 (triiodothyronine), the active hormone that enters cells and regulates metabolic function. TSH (thyroid stimulating hormone) is released by the pituitary gland and is the messenger hormone that tells the thyroid how much T4 to produce.

Thyroid Disease

• Thyroid disease occurs when levels of T4, T3 or TSH are above or below normal limits. If the thyroid isn't producing enough T4/T3 or the pituitary isn't sending enough TSH, it causes hypothyroidism, the condition of lowered thyroid function. If there is too much T4/T3 or TSH in the body, it is known as an overactive thyroid or hyperthyroid.

Types of Thyroid Function Tests

• Several types of thyroid function tests measure current levels of all three of these hormones in the bloodstream. The basic test is the TSH test, which seeks to establish how much stimulation the thyroid is receiving from the pituitary. If TSH comes back normal, then additional tests that specifically measure the amount of inactive T4 and active T3 may be performed to refine the cause of glandular imbalance.

Test Results

• The normal range for TSH in the blood is 0.5-2.0 uU/ml. For a T3 test, the normal function range is 80-180 ng/dl, and for a T4 test normal results are considered 4/6-12 ug/dl. Any thyroid function test results outside of these ranges are indicative of thyroid disease.