The most common cause of apparent inefficiency or resistance to oral therapy with levothyroxine for hypothyroidism is nonadhesion. However, in some subjects in whom the control of hypothyroidism is extremely difficult, levothyroxine bioavailability defects should be considered. We report here the case of a 57-year-old woman with hypothyroidism that was well-controlled for the previous 6 years but suddenly presented with poor hormonal control and abdominal symptoms, despite repeatedly reporting good compliance to therapy. Adequate control of thyroid function was only obtained after intestinal giardiasis was diagnosed and treated.
