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.
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