Washington Manual of Medical Therapeutics
Evaluation of Thyroid Function
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General Principles
The major hormone secreted by the thyroid is thyroxine (T4), which is converted by deiodinases in many tissues to the more potent triiodothyronine (T3). Both are bound reversibly to plasma proteins, primarily thyroxine-binding globulin. Only the free (unbound) fraction enters cells and produces biologic effects. T4 secretion is stimulated by thyroid-stimulating hormone (TSH) from the pituitary gland. In turn, TSH secretion is inhibited by T4, forming a negative feedback loop that keeps free T4 levels within a narrow normal range. Diagnosis of thyroid disease is based on clinical findings, palpation of the thyroid, and measurement of plasma TSH and thyroid hormones.
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General Principles
The major hormone secreted by the thyroid is thyroxine (T4), which is converted by deiodinases in many tissues to the more potent triiodothyronine (T3). Both are bound reversibly to plasma proteins, primarily thyroxine-binding globulin. Only the free (unbound) fraction enters cells and produces biologic effects. T4 secretion is stimulated by thyroid-stimulating hormone (TSH) from the pituitary gland. In turn, TSH secretion is inhibited by T4, forming a negative feedback loop that keeps free T4 levels within a narrow normal range. Diagnosis of thyroid disease is based on clinical findings, palpation of the thyroid, and measurement of plasma TSH and thyroid hormones.
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