Hypothyroidism is caused by any structural or functional derangement that interferes with the production of adequate levels of thyroid hormone . It can result from a defect anywhere in the hypothalamic pituitary thyroid axis. As in the case of hyperthyroidism, this disorder is divided into primary and secondary categories depending on whether the hypothyroidism arises from an intrinsic abnormality in the thyroid or occurs as result of hypothalamic or pituitary disease. The causes of hypothyroidism can be divided in to several categories.

Primary hypothyroidism accounts for the vast majority of cases of hypothyroidism. The most common cause of hypothyroidism in iodine sufficient areas of the world is chronic autoimmune thyroididtis or Hashimoto thyroiditis. This condition is variously reported to cause 15 to 60% of acses of hypothyroidism. Both cellular and humoral factors contribute to the thyroid injury and hypothyroidism in chronic autoimmune thyroiditis.

Hypothyroidism can follow thyroid surgery or radiation be drug induced or occur as a result of an infiltrative disorder. A large resection of the gland for the treatment of hyperthyroidism or excision of primary neoplasm is a common cause of hypothyroidism. The gland may also be ablated by radiation whether in th eform of radioiodine administered for the treatment of hyperthyroidism, or exogenous irradiation such as external radiation thearpy to the neck. Drugs given intentionally to decrease thyroid secretion can cause hypothyroidism as can agents used to treat nonthyroid conditions. Finally infiltratiive disease such hemochromatosis, amyloid and sarcoid are rare causes of hypothyroidism.

Secondary hypothyroidism is that caused by TSH deficiency and teritary (central) hypothyroidism is caused by TRH deficiency. Secondary hypothyroidism can result from any of the causes of hypopituitarism, frequently pituitary tumor other causes include postpartum pituitary necrosis, trauma and nonpituitary tumors as previously discussed. Tertiary (central ) hypothyroidism can be caused by any disorder that damages the hypothatlamus or interferes with hypothalamic pituitary portal blood flow thereby preventing delivery of TRH to the pituitary. This can result from hypothalamic damage from tumors, trauma, radiation therapy or infiltrative disease. Classic clinical manifestations of hypothyroidism include cretinism and myxedema.

Cretinism

Cretinism refers to hypothyroidism developing in infancy or early childhood. The term cretin was derived from the French chretien meaning Christian or christ like and was applied to these unfortunates because they were considered to be so mentally retarded as to be incapable of sinning. In the past this disorder occurred fairly commonly in areas of the world where dietary iodine deficiency is endemic such as the Himalayas inland China and Africa and other mountainous areas. It has become mu8ch less frequent in recent years, owing to the widespread supplementation of foods with iodine. On rare occasions , cretinism may also result from inborn errors in metabolism that interfere with the biosynthesis of normal levels of thyroid hormone.

Myxedema

The term myxedema is applied to hypothyroidism developing in the older child or ender age. The clinical manifestations vary with the age of onset of the deficiency. The older child shows signs and symptoms intermediate between those of the cretin and those of th elder with hypothyroidism. In the elder the condition appears insiderously and may take years to reach the level of clinical suspicion

Clinical features of myxedema are characterized by a slowing a physical and mental activity. The initial symptoms include generalized fatigue; apathy and mental sluggishness which may mimic depression in the early stages of the disease. Speech and intellectual functions become slowed. Patients with myxedema are listless, cold introlerant and frequently overweight. Reduced cardiac output probably contributes to shortness of breath and decreased exercise capacity, two frequent compliants in patientw with hypothyroidism. Decreased sympathetic activity results in constipation and decreased sweating. The skin in these patients is cool and pale because of decreased blood flow.