Effects of Graded Doses of L-Thyroxine on 14C-Leucine Incorporation into Anterior Pituitary Proteins of Thyroidectomized Rats

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Effects of repeated injections of various doses of L-thyroxine (T4) on 14C-leucine incorporation into anterior pituitary proteins of thyroidectomized rats were studied. The normal level of protein-bound iodine (PBI) was maintained by a replacement dose of 0.8μg T4 per 100g body weight per day. Thyroidectomy resulted in a slight increase in 14C-leucine incorporation into total pituitary protein. The increased 14C-leucine incorporation was not suppressed by lower doses of T4 but rather a slight increase was observed. At 2μg dosis or higher doses the T4 suppression was remarkable. Twenty, μg dose of T4 reduced the incorporation below the normal level. There was a significant negative correlation between the rate of 14C-leucine incorporation and PBI values. Disc electrophoresis study revealed that synthesis of growth hormone (GH) decreased significantly after thyroidectomy and this decreased GH synthesis was normalized by repeated injections of 0.8μg of T4. Injections of 20μg of T4 caused no further increase in GH synthesis. Synthesis of prolactin was also diminished by thyroidectomy and restored to normal by T4 supplement. The interrelation between thyroid states and synthesis of anterior pituitary proteins was discussed with special reference to the pituitary-thyroid axis.
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