Abstract
Typically, the role of corticosterone (B) in metamorphosis is considered secondary to that of thyroid hormone, with B having only enhancing effects. In the current study, we demonstrate that the relationship between the thyroid hormones and B is much more complex and that thyroxine (T~4~) may depend on B for some of its functions. Tadpoles of the western toad (Bufo boreas) were treated with various combinations of corticosterone (B), thyroxine (T~4~), triiodothyronine (T~3~), a goitrogen (thiourea; Thio), and a corticoid synthesis inhibitor (metyrapone; MTP). Hormones were extracted from individual tadpoles and whole‐body hormone levels determined by radioimmunoassay. B‐treatment decreased the ratio of T~4~ to T~3~, suggesting that B increased the conversion of endogenous T~4~ to T~3~. In addition, B‐treatment in combination with T~4~ resulted in high whole body‐levels of T~3~. B also caused a decrease in whole body‐thyroid hormone levels (T~4~ and T~3~), suggesting negative feedback on the hypothalamo‐pituitary‐thyroid axis and T~3~ had a similar effect, decreasing whole body‐T~4~ levels. T~4~‐treatment, but not T~3~, increased whole body‐B levels and MTP‐treatment in combination with T~4~ prevented the stimulatory effect of T~4~ on B production. MTP‐treatment alone blocked all steroid metabolism of [^3^H]progesterone by the interrenal in vitro, and lowered whole body‐B levels three‐fold in vivo. Thio‐treatment reduced thyroid hormone levels and also resulted in decreased B. Finally, we suggest that these results demonstrate a system in which T~4~ may regulate its own potency: increasing T~4~ stimulates B production, which increases the conversion of T~4~ to its more active form T~3~. Rising T~3~ levels then cause a decrease in T~4~, removing the stimulatory effect of T~4~ on B. These interactions between the thyroid hormones and B help clarify the mechanisms underlying the morphological effects of these hormones described in the previous study. © 1995 Wiley‐Liss, Inc.