Hypothyroidism results from inadequate production of thyroid hormone, usually because of dysfunction of the thyroid gland due to surgery (thyroidectomy), radiation therapy (particularly with ¹³¹I), inflammation, chronic autoimmune thyroiditis (Hashimoto’s disease) or, rarely, conditions such as amyloidosis and sarcoidosis. It may also result from pituitary failure to produce thyroid-stimulating hormone (TSH), hypothalamic failure to produce thyrotropin-releasing hormone, inborn errors of thyroid hormone synthesis, inability to synthesize thyroid hormone because of iodine deficiency (usually dietary), or the use of antithyroid medications such as propylthiouracil.

In patients with hypothyroidism, infection, exposure to cold, and sedatives may precipitate myxedema coma.

Typically, the early clinical features of hypothyroidism are vague and may include fatigue, forgetfulness, sensitivity to cold, unexplained weight gain, and constipation. As the disorder progresses, characteristic myxedematous signs and symptoms appear, such as decreasing mental stability; dry, flaky, inelastic skin; puffy face, hands, and feet; hoarseness; periorbital edema; upper
eyelid droop; dry, sparse hair; and thick, brittle nails.

Cardiovascular involvement leads to decreased cardiac output, slow pulse rate, signs of poor peripheral circulation and, occasionally, an enlarged heart. Other common effects include anorexia, abdominal distention, menorrhagia, decreased libido, infertility, ataxia, and nystagmus. Reflexes show delayed relaxation time (especially in the Achilles tendon).