Higher BMI and Hypothyroidism Risk Study

A major longitudinal study from Canada has brought new clarity to an area of ongoing debate in women’s health: the relationship between menopause and hypothyroidism. Published in the journal PLOS One, this research concludes that the timing of natural menopause does not increase the risk of overt hypothyroidism, but that higher body mass index (BMI) significantly does.

This revelation has deep implications for clinicians, public health policymakers, and aging women concerned about thyroid dysfunction.

Understanding Hypothyroidism and Menopause

Hypothyroidism is a condition in which the thyroid gland fails to produce sufficient hormones, slowing the body’s metabolism and affecting energy levels, weight, mood, and temperature regulation. It is particularly prevalent in older women, with 70% of cases diagnosed in women over the age of 50.

The link between reproductive hormone changes and thyroid dysfunction has long intrigued scientists. The sharp decline in estrogen during menopause affects multiple physiological processes and has been speculated to influence thyroid hormone metabolism. Prior research has been inconclusive, often limited to subclinical forms of hypothyroidism or lacking the robust, long-term data needed to confirm causality.

About the Study

The new study used data from the Canadian Longitudinal Study on Aging (CLSA), tracking 15,330 postmenopausal women over a period of 10 years, covering a total of 257,589.7 person-years. Researchers set out to determine whether age at natural menopause (ANM) had any influence on the likelihood of developing overt (clinically diagnosed) hypothyroidism.

They excluded women who had not yet reached menopause, those with surgically or medically induced menopause, and anyone diagnosed with hypothyroidism prior to menopause. Women with extreme menopause ages (<40 or >67) were also excluded to avoid skewing the analysis.

The study accounted for various confounding factors, including:

• Age

• Education and ethnicity

• Smoking and alcohol use

• Physical activity levels

• Body Mass Index (BMI)

• Reproductive history (e.g., age at menarche, use of hormone therapy, number of pregnancies)

Self-reported diagnoses and statistical modeling using Cox proportional hazards regression were used to assess the time to hypothyroidism onset following menopause.

Findings: BMI is the Real Risk Factor

The analysis revealed no significant association between ANM and hypothyroidism incidence. Women who experienced early or late menopause were no more or less likely to develop hypothyroidism than others.

However, BMI showed a consistent and significant association:

• Women with a BMI of 25–29 had a 20% increased risk.

• Women with a BMI ≥30 had a 40% increased risk.

Interestingly, this finding also raises the question of directionality—does higher BMI cause hypothyroidism, or does an underactive thyroid lead to weight gain? The authors note that thyroid dysfunction can reduce metabolic rate and potentially lead to increased body weight, suggesting a complex, bidirectional relationship.

Interpretation and Clinical Implications

The findings align with one prior study on subclinical hypothyroidism but contradict others that suggested a link between early menopause and autoimmune thyroid conditions.

One possible explanation lies in estrogen’s role in increasing thyroid-binding globulin (TBG) without significantly altering free T4 levels in healthy women, which could mask or modulate thyroid dysfunction. This might explain why menopause timing, which impacts estrogen levels, showed no influence in this study.

This research should reassure postmenopausal women that early or late menopause is not a predictor of thyroid disease, but it should also encourage healthcare providers to pay close attention to BMI in older women.

Study Strengths and Limitations

Strengths of the study include:

• Large, representative sample size

• Detailed demographic and health data

• Use of longitudinal data across multiple cycles

Limitations include:

• Self-reported hypothyroidism diagnoses and BMI data

• Potential for underdiagnosis due to symptom overlap with aging

• Lack of direct hormone measurements

• Predominantly White population (94.4%), limiting generalizability

Conclusion

In summary, this extensive 10-year Canadian study found that higher BMI is a key modifiable risk factor for hypothyroidism, whereas age at menopause is not. This finding helps sharpen the focus for prevention and treatment strategies, shifting attention toward metabolic health and lifestyle interventions in postmenopausal women.

Healthcare professionals and researchers should now look to incorporate more targeted studies involving hormone assays and deeper investigation into the BMI–thyroid function relationship to build on this compelling foundation.

For more information: Associations between age at natural menopause and risk of hypothyroidism among postmenopausal women from the Canadian Longitudinal Study on Aging (CLSA). Kesibi, D., Rotondi, M., Edgell, H., Tamim, H. PLOS One (2025). DOI: 10.1371/journal.pone.0324635, https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0324635