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INTRODUCTION

Disorders of the thyroid gland result primarily from autoimmune processes that stimulate the overproduction of thyroid hormones (thyrotoxicosis) or cause glandular destruction and underproduction of thyroid hormones (hypothyroidism). Neoplastic processes in the thyroid gland can lead to benign nodules or thyroid cancer.

Thyroidal production of the hormones thyroxine (T4) and triiodothyronine (T3) is controlled via a classic endocrine feedback loop (see Fig. 179-1). Some T3 is secreted by the thyroid, but most is produced by deiodination of T4 in peripheral tissues. Both T4 and T3 are bound to carrier proteins [thyroid-binding globulin (TBG), transthyretin (binds T4 only), and albumin] in the circulation. Increased levels of total T4 and T3 with normal free levels are seen in states of increased carrier proteins (pregnancy, estrogens, cirrhosis, hepatitis, and inherited disorders). Conversely, decreased total T4 and T3 levels with normal free levels are seen in severe systemic illness, chronic liver disease, and nephrosis.

HYPOTHYROIDISM

Etiology

Deficient thyroid hormone secretion can be due to thyroid failure (primary hypothyroidism) or, less commonly, pituitary or hypothalamic disease (secondary hypothyroidism) (Table 181-1). Transient hypothyroidism may occur in silent or subacute thyroiditis. Subclinical (or mild) hypothyroidism is a state of normal free thyroid hormone levels and mild elevation of TSH; despite the name, some pts may have minor symptoms. With higher TSH levels and low free T4 levels, symptoms become more readily apparent in clinical (or overt) hypothyroidism. In areas of iodine sufficiency, autoimmune disease and iatrogenic causes are the most common causes of hypothyroidism. The peak age of occurrence is around 60 years, and prevalence increases with age. Congenital hypothyroidism is present in 1 of 4000 newborns; the importance of its recognition and prompt treatment for child development has led to the adoption of neonatal screening programs.

TABLE 181-1CAUSES OF HYPOTHYROIDISM

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