Pituitary-thyroid interaction: effects of thyroid hormone, non thyroidal illness and various agents on TSH secretion.

Recent developments in thyroid hormone metabolism have helped to understand the complex events which characterize the regulation of TSH secretion. Plasma T3 concentration as well as intrapituitary T3 generation from T4, exert a profound effect on TSH synthesis and release. Pituitary Type II deiodinase differs from Type I deiodinase found in other tissue such as liver and kidney, and in fact different conditions such as hypothyroidism and hyperthyroidism affect these enzymes in opposite direction. Thyroid hormones exert other effects on the pituitary such as increased synthesis of substance P, increased synthesis of GH, and decreased TRH receptors, TRH also modifies its own receptors in the pituitary and exerts modulatory effects on TSH molecule. Patients with non thyroidal illness may display TSH molecules with decreased biological activity. Various agents used in every day praxis may alter TSH and thyroid secretion. The physician must be aware of changes in order to avoid diagnostic pitfalls.