If you are taking high doses of the thyroid medication levothyroxine and the medication is not working properly, it may be because you are lactose intolerant.
This was the surprising news from a study in the medical journal Thyroid 2006, 16(11): 1171-1173. doi:10.1089/thy.2006.16.1171. The study, "Lactose Intolerance Revealed by Severe Resistance to Treatment with Levothyroxine," was by three Spanish researchers, Manuel Munoz-Torres, Mariela Varsavsky, and Guillermo Alonso.
Here's the abstract:
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.
Obviously, it you are taking levothyroxine, you should talk with your doctor just to be safe.