Plasma Thyroxine and Triiodothyronine Levels in Spontaneously Metamorphosing Rana catesbeiana Tadpoles and in Adult Anuran Amphibia*

Abstract

Sensitive and reliable radioimmunoassays [RIA] were used to measure plasma T4 [thyroxine] and T3 [triiodothyronine] levels in spontaneously developing Rana catesbeiana tadpoles at various stages of metamorphosis. During premetamorphosis circulating levels of both T4 and T2 were below the limits of detection of the RIA procedures (T4 < 50 ng/100 ml, T3 < 5 ng/100 ml). A gradual rise in plasma T3 and T4 became apparent during prometamorphosis, and at the onset of metamorphic climax the levels of both T4 and T3 increased sharply. Peak levels for hormones were observed in the middle of metamorphic climax. The circulating T3 level reached a mean peak of 78 ng/100 ml, at least 15 times greater than the level during premetamorphosis. The peak T4 level was 0.5 .mu.g/100 ml, about a 10-fold increase over the premetamorphosis level. The surge in thyroid hormone secretion lasted only for several days, and during the latter half of metamorphic climax there was a fairly rapid decrease in plasma T4 and T3. By 2 days post-climax the levels had declined to about 20% of their peak values. Free T4 and T3 levels in plasma followed the same general pattern as the total hormone levels during the various states of tadpole development. In adult R. catesbeiana, plasma T4 and T3 levels were surprisingly low. Similarly low values were observed in Bufo marinus and in R. pipiens. The very low levels of circulating T4 and T3 both in premetamorphosis tadpoles and in adults suggest that thyroid hormones in anuran Amphibia may be of importance only during the period of metamorphosis.