Effects of prolonged administration of thyrotrophin on serum concentration, release and synthesis of thyroid hormones in mice.

Mice were injected sc with TSH (0.5 U) at 12 h intervals for 5 days. Groups of mice were sacrificed daily to determine serum T4 and T3 concentrations, 4 h thyroidal 125I uptake, distribution of 125I among thyroidal iodoamino acids, and thyroidal content of T4 and T3. Serum T4 and T3 concentrations increased significantly after the initial injection of TSH and gradually decreased thereafter, reaching initial levels on the 3rd and 4th days, respectively. In contrast to serum hormone levels, thyroidal 125I uptake, incorporation of 125I into T4 and T3 increased significantly on the first day and remained elevated throughout the period of TSH-treatment. Thyroidal T4 content expressed as microgram/mg weight of tissue decreased significantly on the first day and thereafter remained constant. Thyroidal T3 content did not change significantly throughout the experimental period. The differences between thyroidal synthesis and thyroidal contents of T4 and T3 strongly suggest that thyroid hormone secretion is being continuously stimulated. Transient increases in serum T4 and T3 concentrations are probably due to a gradual increase in the rate of peripheral degradation of thyroid hormones. These results suggest that TSH-induced refractoriness in thyroidal iodine metabolism does not appear to exist, at least when TSH is given in vivo for 5 days.