Effect of acute exposure to cold on the activity of the hypothalamic-pituitary-thyroid system.

Endocrinology
E HefcoP R Larsen

Abstract

The effects of a sudden but sustained exposure to cold (1 to 6C) on serum TSH, thyroxine (T4) and triiodothyronine (T3) (all measured by radioimmunoassay), pituitary TSH concentration, pituitary TSH secretory responsiveness to hypothalamic extract or synthetic thyrotropin-releasing hormone (TRH) in vitro as well as in vivo, and the changes of the thyrotropin-releasing activity in three TRF-rich hypothalamic areas were determined. In normal animals, serum TSH underwent a series of oscillations, first rising then returning to the basal levels, then rising again, whereas serum T4 and T3 increased within 2 h of cold exposure and remained elevated. Pituitary TSH concentration and the in vitro pituitary responsiveness declined after an initial elevation, whereas the in vivo responsiveness to TRH was diminished throughout the whole exposure to cold. Thyroid-blocked animals with steady, low levels of serum T4 and T3 showed a step by step increase of serum TSH levels and no changes in the other parameters. It is therefore assumed that the decrease of TSH secretion following the initial rise is due to a feedback inhibition by the increased levels of thyroid hormones as is the decreased pituitary responsiveness of TRH in vivo. The pituita...Continue Reading

Citations

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