Effect of metformin on thyroid cancer risk in patients with acromegaly: A preliminary observational study.

PURPOSE: To evaluate the role of metformin on thyroid cancer risk in patients with acromegaly. METHODS: Medical charts of 534 patients with acromegaly that were followed-up between 1983 and 2019 were reviewed. Patients with follow-up duration at least 6 months were included. Cohort entry was defined as first visit date. The date of each case's thyroid cancer diagnosis was defined as index date. Patients were followed until the index date, death, or last visit date, whichever came first. Nested case-control study design was selected to evaluate the association between metformin and the thyroid cancer risk in patients with acromegaly. RESULTS: 291 patients with acromegaly were included into final analysis. The mean age at acromegaly diagnosis was 42.3 ± 1.3 years. The median follow-up duration was 76 [34-132] months. Among 291 patients, 13 patients (4.5%) had thyroid cancer. Thirty-one percent (n = 92) of the patients used metformin for 6 months or longer. One standard deviation (SD) increase in average growth hormone increased the odds of having thyroid cancer by 1.164 folds (p = 0.017). One SD increase of the average insulin-like growth factor 1 to upper limit of normal ratio increased the odds of having thyroid cancer by 1.201 folds (p = 0.004). If a patient used metformin for at least 6 months, the odds to have thyroid cancer was decreased, multiplied by 0.62 with a 95% confidence interval of [0.47, 0.83] (p = 0.0013). The risk of thyroid cancer decreased with increasing duration of metformin use. CONCLUSION: Metformin may decrease the thyroid cancer risk in patients with acromegaly.