Abstract
ABSTRACT Studies on the hypothalamus-pituitary-thyroid axis have been carried out in control and in diabetic male Wistar rats during adaptation to a 125I-labelled low iodine diet. Diabetes was induced by a single streptozotocin injection (7 mg/100 g body weight). Thyroid weight, thyroid 127I and 125I content, [125I]iodoamino acid distribution in thyroid digests, plasma PBI, plasma and pituitary TSH and hypothalamic TRH by radioimmunoassay were measured between 5 and 30 days after saline or streptozotocin injection, corresponding to 15 and 40 days after onset of the 125I-LID. As compared to controls, diabetic animals have smaller thyroids. The 125I and 127I in diabetic thyroids between 5 and 10 days was lower than in control glands; at longer intervals both 125I and 127I increased with time in diabetics, while no remarkable changes were seen in the control glands. Furthermore, the percentage of T3 and the MIT/DIT and T3/T4 ratios in thyroids of diabetic were lower than in the corresponding control group at all intervals. These results are compatible with a deficiency of TSH stimulation and/or with a high thyroidal iodine content. Compared to the controls, each group of diabetic rats presented a significantly lower plasma TSH concentration, in spite of the fact that its plasma PBI is more or similarly reduced in diabetic than in controls. Furthermore, the decreased pituitary TSH secretion in diabetic rats at early intervals is lower than in controls although the pituitary TSH and hypothalamic TRH remain within normal ranges. It is concluded that the decrease in pituitary TSH secretion may be the result of a reduction of pituitary TRH stimulation, secondary to a diminished secretion and/or synthesis of TRH by the hypothalamus.
Published Version
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