Triiodothyronine (T3) and thyroxine (T4) levels in Rana curtipes during development and metamorphosis.

During premetamorphosis, levels of circulating triiodothyronine (T3) and thyroxine (T4) were below the limits of detection of RIA. They became detectable in late prometamorphic stages. A gradual increase in T3 and T4 was observed during this period. A sharp rise in hormone levels was apparent at the onset of metamorphic climax. Peak levels of both hormones were found at Taylor-Kollros stage XXI. The T3 reached a peak level of 101.4 ng/dl, about 3 fold increase over the level at prometamorphosis. Thereafter the circulating hormones (particularly T4) decreased rapidly and reached levels similar to prometamorphic stages. Significant high levels of thyroid hormones (T3 and T4) in metamorphic climax suggest that like other anurans, elevation of these hormones is required for normal metamorphosis in R. curtipes tadpoles.