Long-term alprenolol treatment affects serum T4, T3 and rT3 in euthyroid patients with ischaemic heart disease.

After treatment with alprenolol for 1 year, serum 3,3',5'-triiodothyronine (rT3) was significantly increased (P less than 0.01) in a group (n = 20) of euthyroid subjects compared to a control group (n = 20) given placebo. All subjects had definite or suspected myocardial infarction one year previously. Serum thyroxine (T4), free T3 index (FT4I), serum 3,5,3'-triiodothyronine, (T3) and free T3 index (FT3I) were not significantly different in the two groups. Alprenolol and placebo were gradually withdrawn over 14 days. On the first day after withdrawal a significant decrease in serum rT3 in the alprenolol treated group was the only change observed. Fourteen days after withdrawal a significant fall in serum T4, FT4I, rT3 and a rise in serum T3 and FT3I was found in the alprenolol treated group. Six months after withdrawal the only further change observed in the alprenolol treated group was an increase in T3 and FT3I. No changes occurred in the placebo treated group in any of the hormones studied. The results are consistent with a direct effect of long-term alprenolol treatment on the peripheral levels of serum T4, T3 and rT3 in euthyroid subjects. The changes in the thyroid hormones after withdrawal further indicate withdrawal of a permanent inhibition of 5'deiodinase during long-term treatment with alprenolol in euthyroid subjects.