The concentrations of thyroxine (TJ and triiodothyronine (T,) were determined on samples of fresh sera from adult male and female newts during the course of forelimb regeneration by means of radioimmunoassay (RIA) methods. The results show that the circulatory levels of T, and T3 in males and females do not change significantly from those found in intact control newts as regeneration progresses. Our serum T, values for intact adult newts are low and within the range of levels found by other investigators in various poikilotherms.
Richardson ('401, Schotte and Washburn ('54) and Liversage and Brandes ('77) have shown that following concomitant thyroidectomy and amputation in the adult newt, Notophthalnus viridescens, forelimb regeneration is impeded and abnormal development of skeletal elements ensues, or, in some cases, stumping occurs. But, if thyroidectomized adult newts are given daily injections of thyroxine, beginning ten days post-amputation, regeneration is greatly enhanced (Richardson, '45). In this connection, when Connelly e t al. ('68) added thyroxine to a water medium containing hypophysectomized adult newts they found that limb regeneration was poorly supported. However, when Connelly et al. ('68) and Tassava ('69) applied thyroxine and also injected prolactin limb regeneration was supported more effectively in their hypophysectomized adult newts. More recent studies by Vethamany-Globus and Liversage ('73) have also shown that thyroxine alone does not support regeneration when added as a supplement to organ-cultured adult newt tail blastemata. However, if thyroxine is in combination with insulin and especially when combined with insulin, growth hormone and hydrocortisone (i.e., a more complete hormonal supplement) advanced growth and cartilage differentiation ensued in their adult newt blastemata in vitro. The results mentioned above support previous work from this laboratory that a multiple hormonal background or "hormonal milieu" is essential for appendage regenera-