Comorbidities, Concomitant Medications, and Diet as Factors Affecting Levothyroxine Therapy: Results of the CONTROL Surveillance Project

BACKGROUND The CONTROL Surveillance Project was a comprehensive patient-based survey conducted among hypothyroid patients undergoing treatment. The primary objective of the study was to specifically quantify the prevalence of factors adversely affecting levothyroxine therapy. METHODS Participants were selected from a large proprietary database. Those eligible for the study completed a 21-question survey. RESULTS Of the eligible hypothyroid patients, 925 (92.5%) were being treated with levothyroxine monotherapy. The mean age was 60.4 years; 755 (81.6%) were female and 168 (18.2%) were male. Almost half of those receiving levothyroxine (435, 47.0%) had at least one comorbid condition that could adversely affect its absorption: gastroesophageal reflux disease (33.8% of patients), irritable bowel syndrome (9.7%), lactose intolerance (7.8%), or a history of gastric bypass surgery or bowel resection (3.0%). Other factors reported by many patients that could adversely affect levothyroxine absorption included use of prescription medications (20.6%) and over-the-counter medications (34.3%) used to treat comorbid gastrointestinal (GI) conditions; use of dietary supplements (51.8%, primarily calcium and iron); and intake of foods/beverages high in fiber, iodine, or soy (68.0%). Of the 13.4% who reported difficulty controlling their hypothyroid symptoms, significantly more patients with comorbid GI conditions reported such difficulty (7.8 versus 5.6%, P < 0.01). Frequent changes in levothyroxine dosing (two or more dose changes in the past year) were reported by 8.0% of survey participants. Those with GI comorbidities were nearly twice as likely to have such changes (5.0 versus 3.0%, P < 0.01). CONCLUSION Better initial workup of patients, including identification of relevant GI comorbidities and allergies, may help in the early detection of factors that may affect the performance of levothyroxine.