Levothyroxine Interactions

Magnesium can reduce levothyroxine absorption

Iron reduces levothyroxine absorption by up to 65%

Calcium in dairy binds to thyroid medication, reducing absorption by up to 40%.

Calcium significantly reduces levothyroxine absorption

High-dose biotin (B7) does not directly interact with levothyroxine pharmacologically, but it interferes with biotin-streptavidin immunoassays used for TSH, free T4, free T3, and thyroglobulin. This can produce falsely low TSH and falsely high T4/T3, mimicking hyperthyroidism and leading to inappropriate dose reductions.

Soy protein and isoflavones can bind to levothyroxine in the gut and reduce its absorption, sometimes increasing dose requirements in hypothyroid patients. Case reports and systematic reviews describe rising TSH and unstable thyroid levels in patients consuming soy products close to their dose.

Ashwagandha (Withania somnifera) can increase serum T3 and T4 and lower TSH, effectively adding to the thyroid hormone effect of levothyroxine. Case reports describe patients on stable levothyroxine who developed thyrotoxicosis and even painless thyroiditis after starting ashwagandha.

Dietary and supplemental fiber can adsorb levothyroxine in the gut and reduce its bioavailability, leading to higher TSH and unstable dosing when fiber intake is high or variable. The effect has been demonstrated with high-fiber diets and pharmaceutical fiber supplements such as psyllium.

Tempeh is a fermented soybean cake, and soy protein binds levothyroxine in the gut and reduces its absorption, with documented decreases of 16 percent at 20 grams of soy protein and over 35 percent at 40 grams. Fermentation reduces isoflavone bioavailability but leaves intact soy protein that still interferes with thyroid hormone uptake.

Coffee, including espresso and instant coffee, can reduce levothyroxine absorption by roughly 25 to 55 percent when consumed at the same time as the tablet. Chlorogenic acids and tannins in coffee appear to bind levothyroxine and the acidic environment may also alter dissolution and gastric emptying.

Edamame is whole young soybeans, and the soy protein and isoflavones bind levothyroxine in the gut and reduce its absorption, with measured drops of 16 percent at 20 grams of soy protein and over 35 percent at 40 grams. A one-cup serving of shelled edamame supplies roughly 18 grams of soy protein, enough to cause clinically meaningful interference if eaten close to the dose.

Tofu is made from coagulated soy milk and is one of the most concentrated sources of soy protein in the typical diet, with roughly 20 grams of soy protein per cup. Soy protein binds levothyroxine in the gut and reduces absorption by 16 to over 35 percent depending on dose, and case reports describe TSH rising when tofu is added to a previously stable levothyroxine regimen.

Cabbage releases thiocyanates from glucosinolates that compete with iodide uptake at the thyroid sodium-iodide symporter. Case reports tie very heavy raw cabbage intake (1+ kg/day) to severe hypothyroidism, but typical cooked portions do not measurably affect levothyroxine requirements when iodine is adequate.

Cauliflower contains glucosinolates that can release thiocyanates competing with iodine uptake, theoretically increasing thyroid hormone demand. In practice, a comprehensive systematic review found brassica vegetables at normal dietary intakes do not impair thyroid function when iodine is adequate.