Endocrine Abstracts

Background: The prescription of triiodothyronine (T3) for hypothyroidism is increasing worldwide, however, long-term effects of its use are largely unknown. Previous studies have suggested possible association between T3 use and breast cancer. The aim of the study was to examine the effects of T3 use on cancer incidence and mortality.

Material: Our sample included the full adult population of individuals living in Sweden with at least three purchases of thyroid replacement hormones between July 2005 and December 2017. Individual-level data on drug purchases were linked to register data on cancer incidence and mortality. There were 575,461 individuals with at least three purchases of thyroid replacement hormones, of which 11,147 had at least three purchases of T3 including combinations of l-thyroxine (T4) and T3. Individuals were followed for 7.4 years on average.

Methods: We applied Cox-regression with a time-varying exposure variable, comparing “T3 users” (individuals with at least 3 cumulative purchases of T3) with “T4 users” (the rest). Outcomes included breast cancer incidence, any cancer incidence, all-cause mortality, any cancer mortality, and breast cancer mortality. We adjusted for age, gender, previous thyroid cancer, previous other cancer, use of antithyroid preparations, use of sex hormones, and dosage in multivariate analyses.

Results: Multivariate analyses comparing T3 and T4 users produced a hazard ratio of 1.03 (95% CI 0.83–1.29) for breast cancer incidence (only women), 0.96 (0.85–1.07) for any cancer incidence, 0.92 (0.81–1.05) for all-cause mortality, 0.85 (0.66–1.10) for any cancer mortality, and 1.23 (0.69–2.22) for breast cancer mortality (only women), none of which was significant. Analyses focusing only on new thyroid replacement users (after July 2006) and analyses stratified by sex yielded similar results, with insignificant estimates that were close to one.

Conclusion: In this large Swedish long-term register-based study, the use of T3 compared to T4 did not lead to increased breast cancer incidence, any cancer incidence, all-cause mortality, any cancer mortality, and breast cancer mortality. Other possible negative effects of T3 use were not analyzed in this study and cannot be excluded.