Hypothyroidism

The thyroid gland uses iodine to make thyroid hormones. The two most important are thyroxine (T4), which has four iodine molecules in its chemical structure, and triiodothyronine (T3), which has three. More than 99 percent of all thyroid hormones are bound to proteins in the blood and do not interact with the body cells. Only a small fraction is free, but these free thyroid hormones are the important and active hormones that directly interact with body cells.

Normally, the rate of thyroid production is regulated by the pituitary gland. When there is insufficient thyroid in the body for normal functioning of cells, the pituitary releases another hormone, thyroid stimulating hormone (TSH). This triggers the thyroid to make more hormones. Generally, the more severe the hypothyroid condition, the higher the level of TSH will be.

Hypothyroidism is defined as the clinical and biochemical syndrome resulting from decreased thyroid hormone production. Overt hypothyroidism occurs in 1.5 to 2 percent of women, and 0.2 percent of men, and its incidence increases with age.(2, 3, 4) The vast majority of hypothyroid patients have primary hypothyroidism, or thyroid gland failure. Less common causes include pituitary failure (secondary hypothyroidism) or even more rarely, generalized resistance to thyroid hormone.

Thyroid hormone is essential for normal growth and development during embryonic life. Thyroid hormone deficiency during fetal and neonatal development results in mental retardation. In the child, thyroid hormone deficiency may manifest as growth retardation. In the adult, manifestations are varied and nonspecific. There is slowing of mental activity, as well as cardiovascular, gastrointestinal, and neuromuscular function.(5)

Causes of hypothyroidism include: primary hypothyroidism, Hashimoto's disease, iatrogenic hypothyroidism, iodine deficiency, enzyme defects, thyroid hypoplasia, goitrogens, secondary hypothyroidism, pituitary disease, and hypothalamic disease.