Hypothyroidism can be primary or secondary. In primary hypothyroidism, the hormone deficiency results from a disease or process within the thyroid gland. Hashimoto's disease, an immune-mediated thyroiditis, is an example. In this condition, blockade of thyroid TSH-receptors results in decreased T4 production. Secondary hypothyroidism, which is rare, is characterized by measurably decreased TSH secretion secondary to hypothalamic-pituitary axis impairment.
Pituitary adenoma is the most common cause.
The onset of hypothyroidism is usually gradual. The decreases in thyroid hormone manifest as reduced metabolic rate and sensitivity to endogenous catecholamines. In severe cases, myxedema may occur, presenting with doughy skin, edematous face, lethargy, slow speech, hearing loss, and an enlarged tongue, among other symptoms. Physical examination may reveal an enlarged thyroid gland. A tender thyroid suggests subacute thyroiditis, while a nontender gland is more common in early Hashimoto's disease or postpartum thyroiditis.