Relative Sensitivity of Different Biological Responses to Small Quantities of Thyroxine and Triiodothyronine

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Abstract
Female rats of the Long-Evans strain were thyroidectomized when 28 days of age by a combination of surgery and 131I. Injections of small doses of L-thyroxine (0.01, 0.05, 0.1 and 0.2 Mg) and L-triiodothyronine (0.1 and 0.2 μg) were started immediately and continued for 98 days. Body growth and metabolic rate were followed at regular intervals, whereas hematological values were determined terminally. At autopsy, endocrine and endocrine-dependent organs were removed for weighing and histological examination. Substantial growth was maintained by 0.05 μg of thyroxine without an effect on oxygen consumption. Triiodothyronine was no more effective in maintaining subnormal growth than thyroxine. Furthermore, the 2 hormones possessed the same potency in all other responses measured. Pituitary acidophils showed little numerical increase at doses below 0.2 Mg, but then suddenly increased to 50% of normal at the 0.2 μg doses. Correlation of this sudden increase with differences in growth rate was not possible. As with oxygen consumption, erythropoiesis declined to levels characteristic of hypophysectomy in all thyroidectomized groups, irrespective of treatment. The adrenal cortices of the thyroidectomized rats showed a remarkable sensitivity to thyroid hormones, differing from those of the untreated group even at the 0.01 Mg dose of thyroxine. This dose failed to influence any other biological end-point. Also noteworthy was the responsiveness of the ovaries. They were essentially normal in function at the 0.2 jug doses. It was concluded that, although growth is one of the most sensitive and easily measured responses to thyroxine in thyroidectomized rats, there are others that are equally or more sensitive. (Endocrinology74: 770, 1964)