Abstract
BACKGROUND: Levothyroxine is the most commonly prescribed medication in the United States. Many foods and medications, including calcium supplements, can interfere with levothyroxine absorption. No studies have investigated the effect of cow's milk, a common breakfast staple, on the absorption of oral levothyroxine. Cow's milk contains approximately 450 mg of elemental calcium per 12 oz (355 mL) serving. METHODS: A pharmacokinetic study was conducted in healthy euthyroid subjects to assess levothyroxine absorption with and without concurrent cow's milk consumption. Following an overnight fast, serum total thyroxine (TT4) concentrations were measured at baseline and at one, two, four, and six hours after ingestion of 1000 μg of oral levothyroxine alone or when co-administered with 12 oz (355 mL) of 2% milk. There was a four-week washout period between the two assessments in each subject. RESULTS: Ten subjects (M(age) ± SD = 33.7 ± 10.2 years; 60% male) completed the study. The area under the curve (AUC) of TT4 concentrations was significantly lower when levothyroxine was ingested along with 12 oz (355 mL) of 2% cow's milk (M ± SD = 67.3 ± 12.1) compared to that with levothyroxine alone (73.5 ± 17.0; p = 0.02). Also, peak serum TT4 concentrations were significantly lower when cow's milk was co-administered with levothyroxine (M ± SD = 14.1 ± 0.8 μg/dL) than with levothyroxine alone (13.0 ± 0.9 μg/dL; p = 0.04). CONCLUSIONS: This is the first study to demonstrate that concurrent cow's milk ingestion reduces oral levothyroxine absorption. The findings support previous literature showing the interference of elemental calcium and food with thyroid hormone absorption. Patients managed with thyroid hormone should be advised to avoid taking their levothyroxine simultaneously with cow's milk.