A study published in the journal Thyroid reported on a reason for malabsorption of thyroid medication, and the need for higher-than-normal doses to normalize thyroid function.

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.
Researchers 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.

For more information
Lactose Intolerance Revealed by Severe Resistance to Treatment with Levothyroxine

Systematic appraisal of lactose intolerance as cause ofincreased need for oral thyroxne

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