The authors say that their findings support previous research in this area. When compared to sulfonylurea monotherapy, metformin monotherapy was associated with an increased risk of low TSH (<0.4 mIU/L; incidence rate 79.5/1000 person-years v. 125.2/1000 person-years, respectively; adjusted hazard ratio 1.55, 95% CI 1.09–2.20) in patients with treated hypothyroidism, with the risk appearing highest during the 90-180 days following treatment initiation. No association with suppressed TSH levels (<0.1mIU/L) was observed.
This study is limited by its small size, the lack of information on adherence, and the possibility of residual confounding due to its observational design. The mechanism behind a potential TSH-lowering effect of metformin is unknown and further investigation is required to determine the short- and long-term clinical consequences of this biochemical event, and whether TSH level monitoring is required when starting metformin.