Interaction of thyroxine,energy metabolism and swimming performance of rats

Time required for exhaustive swimming by young-mature rats weighted with lead weights (4% of body weight) was found to decline over a ten week experimental period. While a positive relationship existed between thyroid status and swimming time, a concomitant decrease in swimming time occurred in each of the respective five groups of rats: control (C) rats with intact thyroids, athyroid (A-Tdx), hypothyroid (Hypo-Tdx), euthyroid (Eu-Tdx), and hyperthyroid (Hyper-Tdx) rats between day 70–140. The latter three groups were thyroidectomized (Tdx) and given daily replacement therapy of L-thyroxine (L-T₄) during the experimental period. Epididymal fat pads were found to be almost non-existent in the A-Tdx rats. Serum triglycerides of A-Tdx, Eu-Tdx, and Hypo-Tdx animals were significantly lower than in C. Daily food intakes were also lower in the A-Tdx and Hypo-Tdx groups than in C and Hyper-Tdx groups. Thyroxine insufficiency, the decreasing feed intake, and particularly the ability to carry the increasing absolute weight load during exercise are factors that may be important in explaining the observed decline in work capacity during the ten week experimental period.