The Effects of Propranolol on Thyroxine Metabolism and Triiodothyronines Production in Man

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Abstract
The effects of propranolol on the turnover of thyroxine (T4), 3,5,3′‐triiodothyronine (T3), and 3,3′,5′‐triiodothyronine (rT3) were determined by a non‐compartmental analysis in seven normal men. Fourteen normal subjects were treated with 0.2 mg T4daily, and half of this group (seven) received in addition 80 mg propranolol daily. Fifteen days of propranolol treatment did not alter serum T4concentration or T4turnover. However, it lowered serum T3concentration from 173 to 102 ng/dl (P= 0.001); T3clearance was unchanged. Propranolol treatment elevated serum rT3concentration from 54 to 69 ng/dl (P= 0.05); rT3metabolic clearance rate fell from 105 to 90 liters/day but the difference did not reach statistical significance. The rT3disposal rate was unchanged by propranolol. The fractional T4disposal which was degraded via the deiodinative pathways was reduced from 82.0 per cent in the control subjects to 65.5 per cent in the propranolol treated subjects. Therefore, propranolol appears to be a potent inhibitor of 5′‐deiodination. The interpretation of serum T3measurements in patients treated with propranolol requires caution.