Dependence of body growth on thyroid activity in turtles

Abstract

AbstractWe surgically thyroidectomized (Tx) or sham‐operated hatchling slider turtles—Trachemys (Pseudemys) scripta—and measured their growth (mass and carapace length) for 5 to 7 months. Autopsy at the end of the experiments revealed that up to 30% of the thyroidectomized animals had regenerated thyroid follicles (thyroids had few but enlarged follicles). Therefore, we segregated thyroidectomized turtles a posteriori into two groups: those with observable thyroid tissue at autopsy, greater than 3 ng/ml plasma thyroxine (T4), and normal plasma T4 binding capacity were designated partial Tx (PTx), and others with no observable thyroid tissue, less than 3 ng/ml plasma T4, and low plasma T4 binding capacity were designated Tx. Growth rate was significantly reduced 6–8 weeks after thyroidectomy in the Tx animals; length‐specific growth rate was affected sooner than mass‐specific growth rate. Growth of Tx animals continued to diverge from sham turtles through the end of the experiment; PTx animals did not differ from shams at any time. In a second experiment growth rate was reduced in Tx turtles maintained under both varibale (14L:10D, thermal gradient of 19 to 40°C) and constant (constant light and temperature—30°C) environmental conditions (i.e., reduced growth rate was probably not related to a thermoregulatory dysfunction). However, all animals maintained under the constant conditions stopped growing after 3 months, whereas turtles kept under varibale conditions continued to grow to the end of the experiment (4.5 months). Thyroxine (T4) replacement therapy of long‐term Tx turtles restored growth after 6 weeks.Plasma thyrotropin (TSH) was greatly elevated in Tx turtles (301 ± 48.7 ng/ml) and at low to nondetectable levels in controls (⩾ 1 ng/ml). T4 replacement therapy was effective in reducing TSH levels in Tx turtles. Plasma T4 and TSH concentrations showed the expected inverse relationship. However, although PTx animals had elevated plasma T4 levels that were in the lower range of the sham controls (PTx: 36.6 ± 8.2 vs. 112.7 ± 17.1 ng/ml in sham), their plasma TSH was also significantly elevated (36.5 ± 9.3 ng/ml vs. 0.8 ± 0.2 ng/ml in sham). These results demonstrate the requirement of an intact thyroid gland for normal growth of turtles, and provide the first direct evidence for a role for the thyroid in body growth of a reptile. They also reveal that the set‐point for the feedback between the thyroid and pituitary secretion can be altered by perturbation of thyroid activity.