Regulation of the Pituitary-Thyroid Axis in Man: Relationship of TSH Concentration to Concentration of Free and Total Thyroxine in Plasma

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Abstract
To study thyroid-pituitary interrelationships in man, serial measurements of plasma thyrotropin (TSH), protein-bound iodine (PBI) and free thyroxine have been made in hypothyroid patients during therapy with L-thyroxine. In each patient, PBI and free thyroxine concentrations increased progressively during therapy. Conversely, initially elevated plasma TSH concentrations fell, ultimately becoming undetectable in all. Thus, a curvilinear relationship was found relating plasma TSH to both PBI and free thyroxine. At concentrations of free thyroxine below the normal, plasma TSH rises rapidly with small decrements in circulating thyroid hormone. Above this point, relatively smaller reductions in plasma TSH accompany similar increments in total or free thyroxine levels. The secretion of pituitary TSH appears to be finely regulated over a wide range of plasma thyroid hormone concentrations.

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