BACKGROUND
Hypothyroidism is common. Treatment of overt hypothyroidism, where the TSH is increased and the FT4 is decreased, is clear. Treatment of subclinical hypothyroidism, where the TSH is increased but the FT4 is normal, is less clear. Overt hypothyroidism is associated with heart problems and a greater risk of heart disease and death. Treating patients with overt hypothyroidism will decrease those risks. However, for those with subclinical hypothyroidism, results have been mixed based on thyroid level, level of TSH increase, and patient age. Prior studies of more than 55,000 people found that people with subclinical hypothyroidism and very high TSH levels had a greater risk of heart disease and death. Other studies also found more heart disease in younger adults with subclinical hypothyroidism. However, some studies in older adults did not show that treatment lowered the risk of death.

Some speculate that treating subclinical hypothyroidism may help lower heart risks in younger adults between 40 and 70 years old. In this study, Ran and his team looked at how hypothyroidism affects the risk of death using a very large group of adults in China, including younger adults.

THE FULL ARTICLE TITLE
Ran X, et al. Hypothyroidism and risks of all-cause and cardiovascular mortality: a retrospective cohort study of 70,276 Chinese adults. Thyroid 2025;35(12):1403–1411; doi: 10.1177/10507256251393523. PMID: 41213612.

SUMMARY OF THE STUDY
Data was collected from a large-volume hospital in China from 2017-2022, of patients aged between 25 to 84 years old who underwent routine health examinations. From the 116,267 patients, 70,276 were included, and from those 66,987 (95.3%) had normal thyroid function, 2436 (3.5%) had subclinical hypothyroidism, and 853 (1.2%) had overt hypothyroidism, with an average follow-up of 5.1 years.

Those patients with subclinical hypothyroidism and overt hypothyroidism were older (45 and 48.7 vs. 42 years) and more often female (62% and 66.7% vs. 46.6%) when compared to participants with normal thyroid levels. Other noticeable differences included higher rates of hypertension, diabetes, and dyslipidemia.

Data showed 329 deaths in patients with normal thyroid levels, 16 in patients with subclinical hypothyroidism and 14 in patients with subclinical hypothyroidism. From those deaths, overt hypothyroidism was associated with increased all-cause mortality risk and increased cardiovascular risk. It is important to note that all-cause mortality in those with overt hypothyroidism was higher among participants with diabetes but not among those without diabetes.

On the other hand, patients with subclinical hypothyroidism did not show an increase in mortality risk, but when adjusted for age, those patients <50 years of age had higher all-cause mortality, but the sample size was not reported, and some limitations were noted.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
The study confirmed prior studies that reported having overt hypothyroidism is linked to a higher risk of heart problems and dying, while subclinical hypothyroidism was only associated with a higher risk of mortality for those under 50 years old. However, there were several limitations to the study, and the authors recommended caution in applying these to clinical practice. Overall, more research needs to be conducted in this field before clear recommendations can be applied.

— Joanna Miragaya, MD

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