FLIGHT BEHAVIOR AND THYROID-HORMONE REGULATION IN HOMING PIGEONS

Homing pigeons which were not given flight training for 3 months prior to the experiment, were flown the same distance of 48 km from the usual release site as reported in our earlier studies using pigeons which were in regular training. In these pigeons the flight lasted 90-160 min instead of the usual 60-80 min taken by pigeons which had regular training. This flight produced a more than two-fold increase in plasma levels of reverse triiodothyronine (rT3), concomitant with reductions in thyroxine (T4) and triiodothyronine (T3) levels and also in T3/T4 ratio. The increase in plasma levels of rT3 and the concomitant decrease in levels of T4 and T3 with no change in plasma osmolality, suggest inhibition of T4 secretion and 5'-monodeiodination, and conversion of T4 to rT3. The conversion of T4 to rT3, the inactive form of T3, represents a mechanism of autoregulation of thyroid hormone function during strenuous and extended flight.