Alterations in systolic time intervals in primary hypothyroidism as a consequence of warming

The systolic time intervals (STI) of the preejection period, left ventricular ejection time and their ratio are influenced by thyroid function, the ratio increasing as thyroid secretion diminishes. We have previously shown that the majority of patients with primary hypothyroidism have a subnormal body temperature and that warming such individuals is accompanied by a fall in circulating levels of serum T4, T3, and TSH. On this occasion we studied 24 patients with primary hypothyroidism. Fifteen of these were warmed at rest in bed for 48 h; the remaining 9 control patients were rested for the same period, but not heated. Four euthyroid volunteers were warmed for 24 h, as were a further 5 hypothyroid subjects. Estimations of STI, T4, and T3 were made in all individuals at the beginning and end of the study period. In all warmed hypothyroid patients there was a rise in rectal temperature and a simultaneous increase in the STI. There was a significant fall in serum T4 and T3 levels in those hypothyroid subjects warmed for 48 h, but not in those warmed for 24 h; the latter, however, showed a significant fall in circulating TSH levels. In the 4 warmed euthyroid volunteers there was no significant alteration in STI and there was no change in T4, T3, and TSH levels in the 2 subjects in whom they were measured. Thus, left ventricular function deteriorates with warming in patients with primary hypothyroidism.