How borderline impairment of thyroid function can affect thermoregulation is an important issue because of the antithyroidal properties of a many environmental toxicants. This study compared the efficacy of heat and cold stress to identify thermoregulatory deficits in rats subjected to borderline and overt hypothyroidism via subchronic exposure to propylthiouracil (PTU). After 3 wk of exposure to PTU in the drinking water (0, 2.5, 5, 10, and 25 mg/l), rats were subjected to a heat stress challenge (34°C for 2.5 h). After one more week of PTU treatment, the same rats were subjected to a cold stress challenge (7°C for 2.5 h). Core temperature (T_c) was monitored by radiotelemetry. Baseline T_c during the light phase was reduced by treatment with 25 mg/l PTU. The rate of rise and overall increase in T_c during heat stress was attenuated by PTU doses of 10 and 25 mg/l. Cold stress resulted in a 1.0°C increase in T_c regardless of PTU treatment. The rate of rise in T_c during the cold stress challenge was similar in all PTU treatment groups. There was a dose-related decrease in serum thyroxine (T₄) at PTU doses 5 mg/l. Serum triiodothyronine (T₃) was reduced at PTU doses of 5 and 25 mg/l. Serum thyroid-stimulating hormone (TSH) was marginally elevated by PTU treatment. Overall, heat stress was more effective than cold stress for detecting a thermoregulatory deficit in borderline (i.e., 10 mg/l PTU) and overtly hypothyroid rats (i.e., 25 mg/l PTU). A significant thermoregulatory deficit is manifested with a 78% decrease in serum T₄. A thermoregulatory deficit is more correlated with a reduction in serum T₄ compared with T₃. Serum levels of TSH are unrelated to thermoregulatory response to heat and cold stress.