1Private Practice, 2University of Buenos Aires
Preliminary observations indicated that co-administration of acidic compounds might enhance absorption of levothyroxine. In order to assess whether this effect would be associated with vitamin C administration, we studied 11 patients (8 women and 3 men, age range 28-75 years; mean 45.2 15.4) being treated with levothyroxine at doses of >1.7 g/kg (average 2.24 0.39 g/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, and non-compliance were excluded. During the study period, the patients continued the same dose of levothyroxine but supplemented it with 1 g of vitamin C in 200 cc of water. After six weeks of taking the levothyroxine with vitamin C, serum TSH decreased in all 11 patients (average reduction 68.6 26.8%), and the target or desired level of TSH was achieved in 9/11 patients. The difference between TSH levels before and after treatment with vitamin C was significant: Basal TSH (IFMA) was 8.54 4.33 mIU/L vs. a mean TSH on vitamin C treatment of 1.88 1.50mIU/L (p<0.001).
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.
