Thyroid Function Tests Health Article

Definition

Thyroid function tests are blood tests used to evaluate how effectively the thyroid gland is working.

These tests include the thyroid-stimulating hormone test (TSH), free and total thyroxine tests (FT₄,T₄), the free and total triiodothyronine tests (FT₃,T₃), the thyroxine-binding globulin test (TBG), and the T-uptake test.

Purpose

Thyroid function tests are used to:

• Help diagnose an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism).
• Evaluate thyroid gland activity.
• Monitor response to thyroid therapy.

Thyroid hormones regulate the rate of cellular activity and affect body temperature, appetite, sleep, and mental health. A low level of thyroid hormone results in myxedema. Although the severity of disease may range from very mild to severe, symptoms associated with hypothyroidism are anemia, malaise, intolerance to cold, hyperlipidemia, fluid retention, and depression. A high level of thyroid hormone causes hyperthyroidism. Classical symptoms include insomnia, intolerance to heat, weight loss, and rapid heart rate.

Both hypo- and hyperthyroidism can be caused by several mechanisms. Primary hypo- and hyperthyroidism are caused by conditions intrinsic to the thyroid, while secondary hypo- and hyperthyroidism are caused by pituitary-hypothalmic failure. T₄ is present in much higher concentrations than T₃, but T₃ is physiologically more potent. Thyroid hormones are active only when not protein bound (i.e. as free hormone). Circulating free hormone levels are regulated by pituitary release of thyroid stimulating hormone (TSH). The release of TSH controlled by negative feedback. Increased blood levels of free hormone inhibit pituitary release of TSH.

Precautions

Many drugs affect the results of thyroid function tests without causing thyroid disease. Some common drugs known to depress thyroid hormone levels are dopamine, corticosteroids, lithium, salicylates, anticonvulsants, and androgens. Thyroid hormone levels may be increased by estrogens, clofibrate, and opiates. TSH, TBG, and T-uptake levels are also affected by many of the drugs cited above. In addition, acute and chronic illnesses and pregnancy also affect thyroid function tests. Such conditions may be confused with clinical hypo- or hyperthyroidism. When possible, patients may be requested to discontinue medications that are known to interfere with the tests several days or more prior to testing.

While most drugs that interfere with thyroid function tests do so by altering thyroxine-binding protein concentrations, peripheral conversion of T₄ to T₃, and other in vivo mechanisms, a few substances (mainly heterophile and autoantibodies) may interfere directly with the analysis. Such interference should be suspected by a physician who sees a test result that is inconsistent with the patient's symptoms or other thyroid function test results.