Changes in thyroid activity of Ambystoma gracile (Baird) during different larval, transforming, and postmetamorphic phases

Thyroid activity for two populations of Ambystoma gracile acclimated to laboratory conditions was assessed by thyroid radioiodide (125I) uptake and serum thyroxine level determined by radioimmunoassay. Both populations exhibited a gradual rise in thyroid activity preceding the mean size for metamorphosis. Larval and transformed salamanders also exhibited a decline in thyroid activity following attainment of this mean size, and this decline was observed for both transformed and neotenous animals. Thyroid activity and serum thyroxine levels were greatest during metamorphic climax. Neoteny was attributed to the generally lower thyroid activity preceding the mean size for metamorphosis which was observed for the high altitude population (having greater neotenic tendencies).