Diabetes
Does lower thyroid function raise the risk of type 2 diabetes?
Open access · cc by · source: Europe PMC
People with lower thyroid function, even within the normal range, were somewhat more likely to develop type 2 diabetes, especially if they already had prediabetes.
Study at a glance
- Design
- Cohort — Prospective population-based cohort; baseline TSH and FT4 related to later diabetes with Cox proportional hazards models
- N
- N=8452 · 8452 diabetes-free participants with thyroid function measurements across three Rotterdam Study cohorts; 798 developed diabetes
- Population
- Middle-aged and older adults (mostly white, aged 45+) in Rotterdam, the Netherlands
- Outcome
- Incident type 2 diabetes and progression from prediabetes to diabetes
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Key findings
During follow-up 798 people developed diabetes. Each doubling of TSH (a sign of lower thyroid function) was linked to a hazard ratio of 1.09 for diabetes, while higher FT4 was linked to lower risk, and these patterns held within the normal reference range. The link was stronger in people with prediabetes: comparing the highest with the lowest FT4 tertile, the hazard ratio for progressing to diabetes was 0.63, and absolute risk fell from about 35% to almost 15% across normal FT4 levels.
Methodology
The researchers used the Rotterdam Study, a long-running Dutch population cohort, and selected 8452 adults without diabetes who had blood tests of thyroid-stimulating hormone (TSH) and free thyroxine (FT4). They followed them for an average of about 7.9 years and recorded who developed type 2 diabetes using GP records, pharmacy data and repeated glucose tests. Hazard ratios were adjusted for age, sex, glucose, smoking and, in a second model, for insulin, blood pressure, cholesterol and BMI.
Limitations
This is observational, so it cannot show that low thyroid function causes diabetes; the authors note residual confounding cannot be excluded and a shared genetic predisposition is possible. Thyroid function was measured only once at baseline. Participants were mostly white and aged 45 or older, so results may not apply to younger or more diverse populations. The study does not test whether treating low-normal thyroid function would prevent diabetes.
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