Vitamin C improves the apparent absorption of levothyroxine in a subset of patients receiving this hormone for primary hypothyroidism

As in some patients with hypothyroidism, because of unknown reasons, large doses of levothyroxine are required to achieve a therapeutic endpoint, and preliminary observations have indicated that an hypoacidic gastric environment is associated with a reduction in levothyroxine bioavailability, and that co-administration of vitamin C might enhance absorption of certain drugs, we assessed whether this effect would be obtained taking levothyroxine with vitamin C. Design: We studied 28 patients (24 women and 4 men, age range 26-76 years; mean 48.0 ±17.75) treated with levothyroxine at doses of >1.70 μg/kg but failing to achieve their target TSH level. During the control period, each patient had at least two determinations of TSH indicating inadequate dosage. Interfering factors that could alter levothyroxine absorption such as celiac disease, calcium, iron, or antacid use, among others, and non-compliance were excluded. During the study period, the patients continued on the same dose of levothyroxine but took the tablet with 1 g of vitamin C in 200 cc of tap water, instead of the same volume of water alone. Serum TSH levels were prospectively measured 6-8 weeks after starting co-administration with vitamin C, and two months later. Main outcome: After six-eight weeks of taking levothyroxine with vitamin C, serum TSH decreased in all 28 patients (average reduction 69.79 ±22.19 %), and the target or desired level of TSH was achieved in 19/28 patients. The difference between TSH levels before and after treatment with vitamin C was significant: Basal TSH (IFMA) was 9.01 ±5.51 mIU/L vs. a mean TSH on vitamin C treatment of 2.27 ±1.61mIU/L (p<0.0001). Conclusions: 1) Vitamin C enhances oral absorption of levothyroxine; 2) Co-administration of Vitamin C with levothyroxine should be considered in patients with difficulties in the absorption of levothyroxine. rev Argent Endocrinol Metab 48: 16-24, 2011 No financial conflicts of interests exist.