Stages along the hypothalamus-pituitary-thyroid axis of salamanders at which various species are thought to have blocked metamorphosis. Normally, thyrotropin-releasing factor from the hypothalamus causes the release of thyrotropin from the pituitary. Thyrotropin causes the thyroid gland to synthesize and release T3 and T4. These thyroid hormones bind to their receptors in competent tissues. Ambystoma tigrinum and A. gracile have defects in thyrotropin-releasing hormone, while A. mexicanum has a defect in thyrotropin production. Eurycea, Necturus, and Siren appear to have a receptor defect in the thyroid hormone-responsive tissues. Eurycea will metamorphose when exposed to extremely high concentrations of thyroxine, while Necturus and Siren do not respond to any dose. (After Frieden 1981.)