Nunes M T, Bianco A C
Braz J Med Biol Res. 1986;19(2):311-8.
Chronic exposure of rats to cold (4 degrees C) leads to thyroid gland hyperactivity as a compensatory mechanism for activating body heat production. There is increased extrathyroidal production of T3 from T4 in parallel to thyroid hormone hypersecretion. Since the 5'-deiodination (5'-D) of T4 can be modulated by thyroid hormones, it has been suggested that the increased thyroid hormone secretion may activate the 5'-D enzymatic pathway leading to increased extrathyroidal T3 production. In an attempt to explore this possibility, T4 to T3 conversion was studied in liver and kidney homogenates of thyroidectomized rats which received T4 (0.5 to 50 micrograms/100 g body weight per day) for 10 days. Tissue homogenates were incubated with T4 (5 micrograms) for 2 h and the T3 generated was measured by RIA as an index of the activity of the 5'-D pathway. A direct relationship between T4 dose and the production of T3 by the homogenate was observed. 5'-D activity was significantly decreased in hypothyroid rats and greatly increased in hyperthyroid rats. Thyroidectomized rats treated with a replacement dose of T4 (1 microgram/100 g body weight/day) were exposed to 4 degrees C for 60 days. Despite the absence of the thyroid gland, increased 5'-D activity was observed in both liver and kidney homogenates compared to both intact and T4-treated thyroidectomized rats maintained at 25 degrees C. We conclude that chronic cold exposure of rats stimulates 5'-D activity which is independent of the concomitant thyroid gland hyperactivity.
大鼠长期暴露于寒冷环境（4摄氏度）会导致甲状腺功能亢进，这是激活身体产热的一种代偿机制。甲状腺激素分泌过多的同时，甲状腺外由T4生成T3的过程也会增加。由于T4的5'-脱碘作用（5'-D）可受甲状腺激素调节，因此有人提出甲状腺激素分泌增加可能会激活5'-D酶促途径，从而导致甲状腺外T3生成增加。为了探究这种可能性，我们对接受T4（每天0.5至50微克/100克体重）治疗10天的甲状腺切除大鼠的肝脏和肾脏匀浆中的T4向T3的转化进行了研究。将组织匀浆与T4（5微克）孵育2小时，并用放射免疫分析法测量生成的T3，以此作为5'-D途径活性的指标。观察到T4剂量与匀浆中T3生成之间存在直接关系。甲状腺功能减退的大鼠5'-D活性显著降低，甲状腺功能亢进的大鼠则大幅增加。给甲状腺切除的大鼠注射替代剂量的T4（1微克/100克体重/天），并使其暴露于4摄氏度环境60天。尽管没有甲状腺，但与在25摄氏度环境下饲养的完整大鼠和接受T4治疗的甲状腺切除大鼠相比，肝脏和肾脏匀浆中的5'-D活性均有所增加。我们得出结论，大鼠长期暴露于寒冷环境会刺激5'-D活性，这一过程独立于伴随的甲状腺功能亢进。
