Temperature effects on thyroid function in the lizard Sceloporus cyanogenys

The effects of chronic and acute cold temperature exposure on thyroid function were investigated in groups of adult lizards, Sceloporus cyanogenys. Some animals were placed in constant temperature chambers at 16°C for 2 wk prior to testing, while those exposed to acute cold were placed in a 4°C environment for 0.5 hr preceded by a 14-day acclimation period at 32°C. Various parameters of thyroid function were measured, including total thyroid ¹³¹I uptake, thyroxine synthesis, and protein-bound iodine. Pituitary-thyroid relationships in both thermal situations were indirectly determined by comparing hypophysectomized and/or TSH injected animals to controls. Chronic cold reduced thyroid activity while acute cold stress apparently stimulated pituitary TSH secretion. Though TSH could not increase oxygen consumption in tissue slices from cold-treated animals, injection of this hormone did cause thyroid hormone uptake by peripheral neurons under similar conditions. Furthermore, serum proteins seem to act as peripheral storage depots for thyroxine during periods of cold-induced inactivity. TSH can apparently free the protein-bound hormone under appropriate conditions. A discussion of the possible role of thyroxine in reptilian thermoregulation is included.