St. John's Wort and Levothyroxine Interaction: Reduced Efficacy and Thyroid Imbalance Risks

St. John's wort (Hypericum perforatum) is a potent inducer of cytochrome P450 enzymes (CYP3A4, CYP2C9, CYP1A2) and P-glycoprotein. Levothyroxine is a substrate for these pathways.

Interaction Mechanism

• Enzyme induction → accelerated levothyroxine metabolism → lower serum T4/T3 levels.
• Possible increased intestinal P-gp efflux → reduced absorption.

Clinical Consequences

• Subclinical or overt hypothyroidism (elevated TSH).
• Loss of thyroid control in previously stable patients.
• Need for levothyroxine dose increase (often 25–50% or more).
• Risk of thyroid hormone fluctuation if St. John's wort started/stopped abruptly.

Other Interactions

St. John's wort also reduces levels of SSRIs, oral contraceptives, antiretrovirals, warfarin, digoxin, and many others.

Recommendation

Avoid concurrent use. If St. John's wort is desired, consult your endocrinologist/PCP for close TSH monitoring and dose adjustment. Never self-manage thyroid medication.