Propranolol (Beta-blocker) decreases plasma triiodothyronine (T3) and increases plasma rT3 in a dose-dependent manner due to a decreased production rate of T3 and a decreased metabolic clearance rate of rT3.
Up to 30% reduction in conversion of T4 -> T3 at high doses (above 160 mg/day) slowly over 1-2 weeks.
HOW? Via inhibition of the 5′-monodeiodinase; enzyme that converts thyroxine (T4) to T3.
NOTE: Other Beta-blockers (Metoprolol and Atenolol) can also cause a small reduction in serum T3 concentrations.
REFERENCES
- Wiersinga WM. Propranolol and thyroid hormone metabolism. Thyroid. 1991 Summer;1(3):273-7.
- Wiersinga WM, Touber JL. The influence of beta-adrenoceptor blocking agents on plasma thyroxine and triiodothyronine. J Clin Endocrinol Metab. 1977;45(2):293.
- Perrild H, Hansen JM, Skovsted L, Christensen LK. Different effects of propranolol, alprenolol, sotalol, atenolol and metoprolol on serum T3 and serum rT3 in hyperthyroidism. Clin Endocrinol (Oxf). 1983;18(2):139.