Thyroid hormone-catecholamine interrelationships during exposure to cold.

Basal plasma levels for adrenalin (A), noradrenalin (NA), L-triiodothyronine (T3), and L-thyroxine (T4) were determined in rats with a chronically inserted catheter. The experiments described in this report 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 degrees C and the same holds for the iodide in the 24-h urine after 7 and 14 days at 4 degrees C. The venous NA plasma concentration was increased 6- to 12-fold during the same period of exposure to cold, whereas the A concentration remained at the basal level. During infusion of NA at 23 degrees 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. This paper presents further evidence for a role of the sympathetic nervous system on T4 metabolism in rats at resting conditions.