Levothyroxine (T4)

Levothyroxine (T4) is a medication in the metabolic drug class. Brand names include Levothyroxine (Synthroid, Levoxyl, Euthyrox, Tirosint).

This page is an educational reference for reviewing interaction information when considering Levothyroxine (T4) with another product. Questions about interactions or any medical condition are best discussed with a healthcare professional.

The most important concerns involve reduced levothyroxine absorption or changes in thyroid status. Calcium, Iron, Dietary Fiber (Prebiotics), and Psyllium Husk can bind levothyroxine or alter gastrointestinal handling, potentially reducing thyroid replacement and causing hypothyroidism. These products should not be taken simultaneously with levothyroxine; doses are generally separated by at least 4 hours. Calcium-fortified Collagen products may pose the same absorption concern.

Sea Moss (Irish Moss), Chlorella, and Iodine can alter thyroid hormone synthesis or thyroid status because of their iodine content, potentially changing levothyroxine requirements. Ashwagandha and Bacopa Monnieri may increase thyroid hormone activity, while St. John's Wort has been associated with raised TSH and reduced thyroid-replacement effect, consistent with increased hormone clearance. Quercetin may bind thyroid hormones and interfere with absorption. Vitamin B7 (Biotin) is a separate concern: high-dose biotin (>1 mg) can interfere with thyroid function tests, producing falsely low TSH and falsely elevated T4/T3 and potentially leading to inappropriate dose adjustments.

Other interactions are less established or may require monitoring rather than routine separation. L-Carnitine and Acetyl-L-Carnitine (ALCAR) may reduce the clinical response to levothyroxine by inhibiting thyroid-hormone entry into cells. Aloe Vera may reduce absorption through its laxative effect. Green Tea Extract (EGCG) may reduce absorption, but evidence is limited. Holy Basil (Tulsi) has shown effects in animal research, with uncertain relevance to levothyroxine use in humans.

Tyrosine’s interaction is theoretical and its clinical significance is uncertain. BCAA has no direct evidence of reducing levothyroxine absorption, and any interaction is theoretical. Alpha-Lipoic Acid (ALA) has a theoretical effect on thyroid-hormone handling, but clinically established impairment of absorption has not been demonstrated. People considering these combinations, or making major changes in fiber or iodine intake, can discuss timing and thyroid-function monitoring with a prescriber or pharmacist.