A Comparison of the Early Metabolic Effects of Triiodothyronine and Thyrotropic Hormone in Man*

The early metabolic effects of sodium l-triiodothyronine (T3) and thyrotropic hormone (TSH) were compared within an eight-hour period. Observations were made on 5 healthy volunteer male medical students on each of three occasions after fasting overnight. Each student received TSH (0.44 unit of Thytropar per Kg.) intramuscularly; T3 (0.5 or 1.0 mg.) intravenously as a single dose; and, as a control, intravenous or intramuscular saline in an amount similar to that administered with the TSH or T3. The subjects lay recumbent, dozing and reading at intervals over the eight-hour period. A significantly greater rise in the basal metabolic rate was noted following TSH than following T3 (0.5 or 1.0 mg.). This rise was accompanied by increases in pulse rate and pulse pressure—which were not observed following T3. T3, however, produced a significant increase in the levels of plasma and urinary inorganic phosphate— which was not observed following TSH. The early metabolic effect of TSH was significantly less in a group of 5 hypothyroid subjects maintained with desiccated thyroid. It is concluded that the early metabolic effect of TSH differs from that of T3, but apparently is of thyroid origin. This may possibly be due to the secretion of a rapidly acting hormone distinct from triiodothyronine.