PHYSICAL EXAMINATION

LABORATORY STUDIES

DIAGNOSIS

Hypothyroidism (Hashimoto’s thyroiditis)

PATHOPHYSIOLOGY OF KEY SYMPTOMS

The patient’s presenting symptoms of fatigue, weight gain, and cold intolerance all indicate an abnormally low metabolic rate. The thin skin and brittle hair also indicate impaired protein synthesis. Thyroid hormone is a major hormone controlling basal metabolism. The thyroid panel confirms hypothyroidism.

Thyroid hormone is an amino acid derivative synthesized in the thyroid gland (Fig. 64-1). The thyroid gland consists of numerous follicles containing colloid surrounded by follicular cells. Follicular cells absorb iodide by sodium-coupled transport and accumulate the iodide within the colloid. The follicular cells also synthesize the large protein thyroglobulin and the enzymes necessary to attach the iodide to the tyrosine. Tyrosine residues bind to the thyroglobulin and are iodinated sequentially within the colloid. Each tyrosine can bind up to two iodides and, after iodination, two tyrosine residues join to form the mature thyroid hormones T₃ and T₄. The thyroglobulin bound to thyroid hormone is absorbed back into the follicular cell. The T₃ and T₄ are separated from the thyroglobulin and secreted into the blood.

FIGURE 64–1 Thyroid hormone is synthesized within the colloid of thyroid follicles. The thyroid follicular cells actively transport iodide (I⁻) into the follicle, where it combines with the tyrosine residues attached to the protein thyroglobulin to make T₃ and T₄. The T₃ and T₄ are separated from the protein and secreted into the blood. MIT, monoiodinated tyrosine; DIT, diiodinated tyrosine.