Because thyroid hormones exert regulatory effects on multiple organ systems, thyroid function should be aggressively evaluated and abnormal function treated in patients who require surgery. Thyroid hormones also significantly affect the metabolism of many drugs, and dose adjustments may be required when function is abnormal. Medical consultants performing preoperative evaluations should include clinical assessments of thyroid function and perform confirmatory tests when indicated.

The adult thyroid gland weighs 15 to 20 g, typically consists of two lobes connected by an isthmus, and is located just below the cricoid cartilage. A remnant of the thyroglossal duct, the pyramidal lobe may be noted arising superiorly from the isthmus or medial side of a lobe. Enlargement of the pyramidal lobe indicates a diffuse thyroidal abnormality. The thyroid gland consists of follicles, which are spheres lined by a single layer of cuboidal cells and are filled with a colloid that is composed primarily of thyroglobulin. A rich capillary network surrounds the follicles, explaining why a bruit is sometimes heard over hyperactive, enlarged thyroid glands. Scattered throughout the thyroid are calcitonin-secreting perifollicular cells. Hyperplasic or malignant transformation of these cells does not result in abnormalities of thyroid function.

Inorganic iodide is actively transported from the blood into the follicular cells, immediately oxidized by perioxidase, and is rapidly incorporated into the tyrosine residues of thyroglobulin. These monoiodotyrosine and diiodotyrosine residues couple to form the iodothyronines thyroxine (T4) and triiodothyronine (T3), which are stored in the follicles.