Lactose Intolerance Revealed by Severe Resistance to Treatment with Levothyroxine

The most common cause of apparent ineffectiveness or resistance to treatment with oral levothyroxine (LT₄) is the result of noncompliance, known as pseudomalabsorption. However, an abnormality in the bioavailability of LT₄ should also be considered in patients requiring large doses of LT₄ to achieve euthyroidism. The incidence of lactose intolerance in Caucasian adult patients is 7%–20%, but the association with resistance to treatment with oral LT₄ is unusual. We report a 55-year-old woman in whom treatment LT₄ for hypothyroidism was found related to a previously undiagnosed oligo-symptomatic lactose intolerance, an unusual association. Although rare, intolerance to lactose should be considered in the differential diagnosis of gastrointestinal diseases that can cause malabsorption of LT₄. The possibility of correcting this disorder with simple dietary measures justifies its consideration.