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), the thyroxine test (T₄), the triiodothyronine test (T₃), the thyroxine-binding globulin test (TBG), the triiodothyronine resin uptake test (T₃RU), and the long-acting thyroid stimulator test (LATS).

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 treatment must be stopped one month before blood is drawn for a thyroxine (T₄) test.Steroids, propranolol (Inderal), cholestryamine (Questran), and other medications that may influence thyroid activity are usually stopped before a triiodothyronine (T₃) test. Estrogens, anabolic steroids, phenytoin, and thyroid medications may be discontinued prior to a thyroxine-binding globulin (TBG) test. The laboratory analyzing the blood sample must be told if the patient cannot...