Thyroid hormone-catecholamine interrelationships during exposure to cold

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Abstract
Basal plasma levels for adrenaline (epinephrine, A), noradrenaline (norepinephrine, NA), L-triiodothyronine (T3) and L-thyroxine (T4) were determined in rats with a chronically inserted catheter. The experiments were started 3 days after the surgical procedure when T3 and T4 levels had returned to normal. Basal levels for the catecholamines were reached already 4 h after the operation. The T3/T4 ratio in plasma was significantly increased after 3, 7 and 14 days in rats kept at 4.degree. C and the same holds for the iodide in the 24 h urine after 7 and 14 days at 4.degree. C. The venous NA plasma concentration was increased 6- to 12-fold during the same period of exposure to cold; the A concentration remained at the basal level. During infusion of NA at 23.degree. C the T3/T4 ratio in plasma was significantly increased after 7 days compared to pair-fed controls, and the same holds for the iodide excretion in the 24 h urine. Evidently the sympathetic nervous system plays a role in T4 metabolism in rats at resting conditions.

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