Drugs That Interact With Levothyroxine

An Observational Study From the Thyroid Epidemiology, Audit and Research Study (TEARS)

Savannah A. Irving; Thenmalar Vadiveloo; Graham P. Leese


Clin Endocrinol. 2015;82(1):136-141. 

In This Article


In Tayside, 10 999 residents over the age of 18 had received at least three consecutive thyroxine prescriptions, within a six-month period, and were alive between 1st January 1993 and 31 December 2012. The mean age of this population was 58·1 years, 8977 (81·6%) were female, and 1311 (11·9%) also suffered from diabetes as a comorbidity. Of these 10 999 patients, 6482 patients were concurrently prescribed a study drug and 3809 patients were on a constant dose of thyroxine for at least two years.

During 1 year prior to the study drug (Table 2), there was no significant change in TSH for any group with one exception. Prior to starting oestrogen, patients in this group had a mean decrease in TSH concentration of 1·47 mU/l (P = 0.008), despite being on a constant dose of thyroxine.

Table 3 (n = 6482) shows the results for all patients on thyroxine who were started on a study drug. Both the median values and interquartile ranges for baseline TSH and for TSH at least 6 months after commencing the study drug were calculated. The number of patients in each analysis varied from 96 patients on disease modifying antirheumatic drugs (DMARDs) to 1944 patients on statins. The baseline TSH used was the most recent serum TSH concentration before the study drug was initiated. An increase in serum TSH from baseline was statistically significant in four study drugs: iron, PPI, calcium and oestrogen (Table 3). A decrease in serum TSH was statistically significant in patients taking statins. There was no statistically significant change for patients taking glucocorticoids, H2 receptor antagonists or DMARDs.

A subset of patients (n = 3809) on a constant dose of levothyroxine for at least 2 years was also analysed (Table 4). The percentage of patients that had a clinically significant increase or decrease in their serum TSH of 5 mU/l is shown. Of these, the same four drugs showed a statistically significant increase in TSH concentration. There was an increase in TSH concentration of < 5 mU/l in 7·5% of patients taking iron, 5·6% of those taking PPIs, 4·4% of those taking calcium and 4·3% of those on oestrogens. There was a significant decrease in TSH concentration of < 5 mU/l in 3·7% of patients starting on statins. The results were not significant for those patients taking glucocorticoids, H2 receptor antagonists or DMARDs.