Interdependence of corticosterone and thyroid hormones in toad larvae (Bufo boreas). II. Regulation of corticosterone and thyroid hormones.

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 (T4) 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 (T4), triiodothyronine (T3), 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 T4 to T3, suggesting that B increased the conversion of endogenous T4 to T3. In addition, B-treatment in combination with T4 resulted in high whole body-levels of T3. B also caused a decrease in whole body-thyroid hormone levels (T4 and T3), suggesting negative feedback on the hypothalamo-pituitary-thyroid axis and T3 had a similar effect, decreasing whole body-T4 levels. T4-treatment, but not T3, increased whole body-B levels and MTP-treatment in combination with T4 prevented the stimulatory effect of T4 on B production. MTP-treatment alone blocked all steroid metabolism of [3H]progesterone by the inter-renal 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 T4 may regulate its own potency: increasing T4 stimulates B production, which increases the conversion of T4 to its more active form T3.(ABSTRACT TRUNCATED AT 250 WORDS)