Diabetes mellitus and thyroid disease often occur together

A Nature Reviews Endocrinology review outlines several reasons for the overlap between diabetes mellitus and thyroid dysfunction, including detection bias, shared risk factors (ageing, obesity, insulin resistance) and possible effects of endocrine-disrupting chemicals. The article also notes that TSH elevations seen in people with obesity often fall after substantial weight loss and that the link differs between T1DM (autoimmune) and T2DM (metabolic/insulin-resistance related).

Researchers describe multiple reasons why diabetes mellitus and thyroid dysfunction commonly co-occur. These include increased screening after one diagnosis, shared risk factors such as ageing, obesity and insulin resistance, and possible contributions from environmental endocrine-disrupting chemicals. The review highlights that the autoimmune link with thyroid disease is clear for T1DM, while mechanisms in T2DM may involve metabolic and tissue-level thyroid hormone resistance. It also reports that modest TSH elevations in people with obesity frequently normalize after substantial weight loss. What the review reports: - Detection bias from heightened screening can inflate apparent co-occurrence; a large population study cited found a strong association with T1DM but not with T2DM in that setting. - Ageing, obesity, metabolic syndrome and insulin resistance are shared risk factors that can cluster with both T2DM and thyroid dysfunction. - TSH is often modestly elevated in people with obesity, and studies show TSH declines after weight loss in many patients, suggesting an adaptive response rather than primary thyroid failure. - Evidence links insulin resistance to altered thyroid hormone action at tissue level and to correlations between insulin-resistance markers and higher TSH; insulin-sensitizing drugs have been associated with lower TSH in some contexts. - T1DM is linked to autoimmune thyroid disease, while some people with T2DM show immune changes and low-grade inflammation that could contribute to thyroid dysfunction. - Several endocrine-disrupting compounds have been associated with both diabetes and thyroid effects, but causal relationships remain unproven. Summary: The review presents the overlap between diabetes mellitus and thyroid dysfunction as multifactorial, involving screening patterns, shared metabolic risk factors and possible environmental exposures. It emphasizes that elevated TSH in obesity may often reflect adaptive physiology rather than primary thyroid disease. Further research is needed to clarify causal pathways and to better inform epidemiology and clinical approaches. Undetermined at this time.