ABNORMAL THYROID FUNCTION
Inadequate secretion of thyroid hormone during fetal and neona-tal development results in stunted physical and mental growth (cretinism) because of general depression of metabolic activity. In adults, hypothyroidism manifests as lethargy, slow mentation, and generalized slowing of body functions.
Oversecretion of thyroid hormones (hyperthyroidism) is man-ifested by a greatly increased metabolic rate. Many of the other characteristics of hyperthyroidism result from the increased re-sponse to circulating catecholamines (epinephrine and norepi-nephrine). Hypothyroidism and hyperthyroidism are discussed in detail in the following sections.
Oversecretion of thyroid hormones is usually associated with an enlarged thyroid gland (goiter). Goiter also commonly occurs with iodine deficiency. In this latter condition, lack of iodine re-sults in low levels of circulating thyroid hormones, which causes increased release of TSH; the elevated TSH causes overproduc-tion of thyroglobulin and hypertrophy of the thyroid gland. The term euthyroid refers to thyroid hormone production that is within normal limits.