In primary hypothyroidism, thyroid-stimulating hormone (TSH) levels are elevated. In primary hyperthyroidism, TSH levels are low. Elevated or low TSH in the context of normal free thyroxine is often referred to as subclinical hypo- or hyperthyroidism, respectively. In hyperthyroidism, both thyroxine (tetraiodothyronine; thyroxine: T4) and T3 levels (total and free) are usually elevated, but in a small subset of hyperthyroid patients (T3 toxicosis) only T3 is elevated. Free thyroxine (fT4) comprises a small fraction of total thyroxine. The fT4 is available to the tissues and is, therefore, the metabolically active fraction. Elevations in fT4 cause hyperthyroidism, while decrease causes hypothyroidism. Disorders of the thyroid gland are frequently caused by autoimmune mechanisms with the production of autoantibodies known as anti-TPO antibodies. Another useful, yet underutilized, test for thyroid function is reverse T3. Reverse T3 is, as its name suggests, a thyroid hormone similar to T3, except for the placement of the iodine atoms in its aromatic rings. This “reversal” of the placement of the iodine atoms causes reverse T3 to be unusable by the body as a thyroid hormone. Reverse T3 is created by degradation of the T4 under specific circumstances like high stress, starvation, and hepatic dysfunction and severe infections so measuring this non active thyroid hormone gives a more complete picture of what is happening within the body that is contributing to thyroid dysfunction.
