Hypothyroidism, or an underactive thyroid gland, develops when the thyroid fails to produce or secrete as much thyroxine (T4) as the body needs. Because T4 regulates such essential functions as heart rate, digestion, physical growth, and mental development, an insufficient supply of this hormone can slow life-sustaining processes, damage organs and tissues in every part of the body, and lead to life-threatening complications.

There are primary and secondary forms of the condition:

• Primary form occurs most commonly after radioactive iodine therapy or surgery for hyperthyroidism (see Diagnostic Code: 7900 Hyperthyroidism). The next most common type of this form is usually a result of autoimmunity, and is associated with circulating antithyroid antibodies.

• Secondary form occurs most often when there is failure of the hypothalamic-pituitary axis due to deficient thyrotropin-releasing hormone (TRH) secretion from the hypothalamus or lack of thyroid-stimulating hormone (TSH) secretion from the pituitary.

The facial expression is dull, the voice is hoarse, speech is slow, and facial puffiness and swelling develop around the eyes. The patient may have a decreased tolerance to cold. The eyelids may droop, and hair may become thin, coarse, and dry. The skin also becomes dry, scaly, and thick. Weight gain is modest due to a decrease in the metabolism of food and fluid retention. Patients may demonstrate forgetfulness or other evidence of brain impairment, with a gradual change in personality. Some patients may become depressed or demonstrate distinct psychosis, e.g., myxedema madness. Other signs and symptoms include: carotene in the blood (carotenemia) marked by yellowing of the skin (pseudojaundice), enlargement of the heart, constipation, and paresthesia in the hands and feet. Women with hypothyroidism often develop excessive bleeding during menses. A body temperature below normal is common, and mild anemia is also often present.

In primary hypothyroidism, thyroid-stimulating hormone (TSH) levels are elevated. Laboratory data reveal a low level of circulating TSH in secondary hypothyroidism. Other pituitary hormone levels and the levels in their analogous tissues may be low in secondary hypothyroidism. The thyrotropin-releasing hormone (TRH) test may be useful in differentiating between secondary hypothyroidism caused by pituitary failure or hypothyroidism caused by hypothalamic failure. When hypothyroidism is caused by hypothalamic failure, TSH is released in response to TRH. The thyroid hormone levels of T3 and T4 are also evaluated when diagnosing hypothyroidism. In more severe hypothyroidism, both serum T3 and T4 levels are decreased.

The aim of treatment is to return hormone levels to normal range. A variety of thyroid hormone replacements are available for treatment. The preparations are synthetic as well as originating from animals.

A life-threatening complication of hypothyroidism is myxedema coma. It is characterized by a milieu of long-standing hypothyroidism, coma with extremely low body temperatures, absence of reflexes, seizures, carbon dioxide retention, and respiratory decline. Rapid diagnosis is critical because early death is likely.

• Consider service connection on a presumptive basis as a chronic condition (38 CFR §3.307; §3.309 (a) ; §3.309 (e))     

• Consider service connection on a presumptive condition due to herbicide exposure under PL 116-283, William M. (Mac) Thornberry National Defense Authorization Act for Fiscal Year 2021.