The preferential role of triiodothyronine in the regulation of basal metabolic rate in hyper- and hypothyroidism.

The free triiodothyronine index (FT3I) was significantly correlated to basal metabolic rate (BMR) in hyperthyroid (r=+0.63, p less than 0.01) and hypothyroid patients (r=+0.61, p less than 0.05). Elimination of the effect of the free thyroxine index (FT4I) on the total correlation between BMR and FT3I by partial correlation analysis gave partial r=+0.60, p less than 0.01 in hyperthyroid patients and partial r=+0.43, p less than 0.1 in hypothyroid patients. The FT4I did not correlate to BMR in either hyper- or hypothyroid patients. These results point to triiodothyronine as the major regulator of BMR in hyper- and hypothyroidism.