Concentrations of triiodothyronine, growth hormone, and luteinizing hormone in the plasma of thyroidectomised fowl ( Gallus domesticus)

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Surgical thyroidectomy increased ( P < 0.05) the basal concentrations of growth hormone (GH) and luteinizing hormone (LH) in the plasma of 10- to 12-week-old domestic fowl. The administration of thyrotrophin releasing hormone (TRH) (100 μg, sc) increased ( P < 0.01) the GH concentration in both intact and thyroidectomised birds. The magnitude of the TRH-induced increase in GH level was greater ( P < 0.01) in thyroidectomised birds than in intact controls. Although TRH had no effect on LH secretion in the controls, it induced a small ( P < 0.05) rise in the plasma LH level in thyroidectomised birds. In both the intact and thyroidectomised birds the LH concentration was enhanced ( P < 0.05) following the administration of LH-releasing hormone (LH-RH) (20 μg, sc). The increase in the LH level by LH-RH in the thyroidectomised birds was greater- ( P < 0.001) than that in the intact controls. Plasma GH concentrations were unaffected by LH-RH treatment. These results suggest that thyroid hormones inhibit the secretion of LH and GH in birds. In thyroidectomised birds low levels of immunoreactive triiodothyronine (T 3)-like material were measurable in the circulation, despite the absence of regenerated thyroid tissue. The administration of TRH (100 μg, sc) did not enhance the plasma level of this material in thyroidectomised birds, whereas plasma T 3 concentrations were enhanced in intact birds following TRH treatment. These results suggest that the T 3 immunoreactive substance in thyroidectomised birds is extrathyroidal in origin.