a deficiency in hormone production by the thyroid gland.

Hypothyroidism usually results from a disorder of the thyroid gland (primary hypothyroidism), but it can also result from deficiency of thyrotropin (thyroid-stimulating hormone, TSH), the hormone of the anterior pituitary gland that controls the thyroid (central hypothyroidism). Central hypothyroidism may be caused by pituitary disease or deficiency of thyrotropin-releasing hormone, the hypothalamic hormone that maintains thyrotropin secretion.

Primary hypothyroidism has many causes. Congenital primary hypothyroidism is caused by lack of or abnormal development of the thyroid in utero and inherited defects in the synthesis of thyroid hormone. The causes of acquired primary hypothyroidism include iodine deficiency ( iodine is an essential constituent of thyroid hormone and is available only from the diet), chronic autoimmune thyroiditis ( Hashimoto disease), treatment of hyperthyroidism or thyroid cancer with radioactive iodine or surgery, and several drugs. The latter include drugs rich in iodine such as amiodarone (a drug for treating heart rhythm disorders) and lithium carbonate. The thyroid gland may be small, normal-sized, or enlarged.

The clinical manifestations of hypothyroidism are characterized by slowing of most body functions. Neuromuscular symptoms include slowing of thought, speech, and action; lethargy and fatigue; sleepiness; muscle aches and weakness; and slow reflexes. Other common symptoms are dry skin and hair, decreased perspiration, puffy eyes, cold intolerance, deepening of the voice, decreased appetite but a tendency to gain weight, constipation, and irregular menstrual periods and increased menstrual blood flow in women. Cardiac contractility and heart rate decrease as a result of hypothyroidism. Hypothyroidism also raises serum cholesterol concentrations. In very young children hypothyroidism causes mental retardation (intellectual disability), and in children of all ages it causes growth retardation. The most severe form of hypothyroidism, myxedema coma, is characterized by coma, low body temperature ( hypothermia), low blood pressure, and respiratory depression and may be fatal.

Hypothyroidism is diagnosed on the basis of the symptoms and signs described above, and the diagnosis is confirmed by measuring serum thyroxine (the thyroid hormone produced in greatest quantity) and thyrotropin. Measurements of free serum hormone detect hormones that are not bound to proteins in the blood and therefore circulate freely through the body. Measurements of total serum hormone detect hormones that are bound to protein in the blood and hormones that are free. These measurements are significant because free thyroid hormones are metabolically active, whereas hormones bound to proteins are not. In hypothyroidism the usual findings are low serum total and free thyroxine concentrations and high serum thyrotropin concentrations. An exception is that serum thyrotropin concentrations are normal or low in patients with hypothyroidism caused by hypothalamic or pituitary disease.

Patients with hypothyroidism are treated with synthetic thyroxine.