Hypothalamic control of thyrotrophin secretion*

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Abstract
By means of the hemagglutlnation-lnhlbltlon technique, it has been possible to measure the plasma and pituitary levels of TSH following unilateral and bilateral electrolytic lesions placed in the supraoptic area of the hypothalamus of female rats. Unilateral lesions after 8 and 48 hr. caused a temporary decrease in the percentage of circulating TSH followed by a return to normal. Bilateral lesions caused a fall in plasma TSH to a level 41% below normal at 10 days whereas the pituitary level increased 70%. Rats with bilateral symmetrical supra-optic lesions, kept for 3 days at 4[degree] C, had a lower plasma TSH content (23%) and a higher pituitary TSH content (16%) than the sham operated control animals but the levels of pituitary TSH did not show any significant difference (0.100 > P> 0.050). Thyroldectomlzed rats with bilateral lesions kept at 27[degree] C for 3 days had a much lower plasma TSH (39%) and a slightly higher pituitary TSH as compared to the normal intact animals, suggesting that the hypothalamus also influences the synthesis of TSH in the pituitary. Thyroidectomlzed rats with bilateral lesions kept at 4[degree] C for 3 days showed both a plasma and pituitary TSH increase compared to controls at 26[degree] C, suggesting that when a higher demand for thyroid function is present the pituitary gland has some autonomy for both secretion and release of TSH. This autonomy appears to be slight, as there was no statistically significant difference between the pituitary TSH levels of the thyroldectomlzed animals bearing similar supraoptic lesions and exposed to 4 and 26[degree]C.