D-propranolol and DL-propranolol both decrease conversion of L-thyroxine to L-triiodothyronine.

The effects of propranolol (DL-propranolol) and D-propranolol on thyroid hormone metabolism were studied in six euthyroid volunteers receiving L-thyroxine (T4) and six hypothyroid patients receiving T4 replacement. D-propranolol as well as propranolol decreased L-triiodothyronine (T3) concentrations and the ratio of T3 to T4 in the euthyroid subjects, and D-propranolol decreased these variables in the subjects with hypothyroidism (propranolol was not given to this group). It is concluded from this study and from parallel invitro investigations that the effect of propranolol on the conversion of T4 to T3 is unrelated to its beta-adrenergic blocking activity, and that at low therapeutic doses propranolol may exert appreciable "membrane-stabilising" effects in vivo.