Researchers from Erasmus University Medical Center and Harvard T. H. Chan School of Public Health conducted a population-based prospective cohort study, the Rotterdam study, to understand the correlation between thyroid levels and the incidence of diabetes mellitus. The study looked at patients with prediabetes or without diabetes and low thyroid function to determine the incidence of progression to type 2 diabetes mellitus. The researchers noted, “There are no other studies addressing the relation between diabetes and thyroid function in the euthyroid range or in individuals with prediabetes.” The results of this study, they said, can be used as a baseline for further studies as well as provide new insights on this relationship.

The Rotterdam Study consisted of 8,452 patients with a mean age of 64.9 years without diabetes at baseline and followed them until the patients received a diagnosis of diabetes, death, or until January 1, 2012, whichever occurred first. The researchers measured thyroid-stimulating hormone (TSH) levels, free T4 levels, thyroid autoantibodies (TPOAb), and serum glucose to determine incidence along with other confounders or intermediate factors, such as blood pressure, fasting serum insulin, cholesterol levels, body mass index, etc.

During the follow-up period of 7.9 years with 99.4% of participants completing follow-up, the study found that 798 out of 8,452 individuals developed diabetes. The authors reported that for every doubling of TSH levels, the risk of developing diabetes in patients without diabetes was 1.16 times higher (HR 1.06, 95% CI, 1.00-1.13). Also, for every doubling of TSH levels, the risk of prediabetic patients progressing to diabetes was 1.13 times higher (95% CI, 1.03-1.24).

According to the study “higher TSH levels and lower FT4 levels are associated with an increased risk of diabetes and progression from prediabetes to diabetes.” The study authors concluded that even though their finding suggests that low or low-normal thyroid function is associated with increased risk of diabetes, future research is needed to confirm the findings of this study. Furthermore, Dr. Layal Chaker, MD, a lead study author, stated, “Findings suggest that screening in people with prediabetes could be considered for further research.”

Currently, it is difficult to assume if a decrease in thyroid function leads to diabetes, but this study provides a better understanding of the impact that thyroid function can have on the incidence of diabetes.

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