Effects of thyroxine and cold exposure on hypothalamic TRH levels in rats with various pituitary-thyroid states.

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  • UTSUMI MASAFUMI
    The Second Department of Internal Medicine, Kobe University School of Medicine
  • MAKIMURA HIROYUKI
    The Second Department of Internal Medicine, Kobe University School of Medicine
  • TATEIWA MAKOTO
    The Second Department of Internal Medicine, Kobe University School of Medicine
  • SAKODA MASAHIRO
    The Second Department of Internal Medicine, Kobe University School of Medicine
  • BABA SHIGEAKI
    The Second Department of Internal Medicine, Kobe University School of Medicine

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  • Effects of Thyroxine and Cold Exposure

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The hypothalamic content and concentration of thyrotropin-releasing hormone (TRH) were determined by radioimmunoassay in normal, thyroidectomized, hypophysectomized and cold-exposed rats treated with or without thyroxine.<BR>In normal animals, the single administration of thyroxine (1, 5 and 20μg/100g B. W.) altered neither the content nor the concentration of TRH in the hypothalamus. However, seven days' administration of this hormone resulted in the dose-dependent increase in the hypothalamic TRH levels. In thyroidectomized rats, the hypothalamic TRH levels were slightly reduced in spite of the marked increase of plasma TSH levels and decrease of pituitary TSH levels. In the animals given thyroxine (10μg/100g B. W.) for 7 days in addition to thyroidectomy, however, the TRH levels exceeded that in the animals which underwent thyroidectomy alone. The hypothalamic TRH levels were markedly reduced in hypophysectomized rats. Conversely, in hypophysectomized rats given 7 days' thyroxine (1 and 5μg/100g B. W.), the levels were increased dose-dependently. In cold-exposed rats, the plasma TSH levels roughly doubled, but the TRH levels remained unchanged.<BR>These findings strongly suggest that the feedback site of thyroxine extends not only to the pituitary gland but also to the hypothalamus, and that thyroxine has an increasing effect of the hypothalamic TRH level, though the mechanism (s) remain to be clarified.

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