Does lower thyroid function raise the risk of type 2 diabetes?
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.
Source
Thyroid function and risk of type 2 diabetes: a population-based prospective cohort study
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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What they did
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.
What they found
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.
The limits
What it doesn't show
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.
Key terms
- TSH (thyroid-stimulating hormone)
- A pituitary hormone that rises when thyroid output is low, so higher TSH usually signals lower thyroid function.
- Free thyroxine (FT4)
- The unbound, active form of the main thyroid hormone circulating in blood.
- Prediabetes
- Blood glucose that is above normal but below the threshold used to diagnose diabetes.
- Hazard ratio
- How much faster an event occurs in one group or per unit of exposure compared with another, over follow-up time.
- Euthyroid / normal range
- Thyroid test values within the laboratory reference range, i.e. no diagnosed thyroid disease.
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Quiz yourself
In this cohort, higher TSH at baseline was associated with:
Common questions
Why does the normal-range finding matter?
It suggests diabetes risk varies continuously with thyroid function, so even people told their thyroid is 'normal' may differ in risk.
Should people with prediabetes be treated with thyroid hormone?
Not on this evidence; the study only shows an association and the authors call for trials of screening and treatment before any recommendation.
Why was the effect bigger in prediabetes?
People with prediabetes have a high baseline risk, so a modest relative increase translates into a large absolute difference in who progresses.
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