BACKGROUND
Thyroid-stimulating hormone (TSH) is produced by the pituitary, a small gland at the base of the brain that helps regulate many of the body’s hormone systems. TSH regulates the thyroid gland, stimulating it to grow and produce the thyroid hormones T4 (thyroxine) and T3 (triiodothyronine). An abnormal TSH blood level is often the first sign of a thyroid problem. A high TSH level usually indicates that the thyroid is not producing enough thyroid hormone, a condition known as hypothyroidism. In contrast, a low TSH level usually indicates that the thyroid is producing too much thyroid hormone, a condition known as hyperthyroidism.

Researchers have long observed that thyroid blood test results, particularly TSH and free T4 (FT4), may change with age. For example, TSH levels often rise modestly in older adults. However, it remains unclear the magnitude of this change and, more importantly, how much of it reflects normal aging and how much represents true thyroid disease. This uncertainty can make treatment decisions challenging. For example, clinicians may be unsure whether an older adult with a mildly elevated TSH level should be treated with thyroid hormone replacement for hypothyroidism.

This study aims to understand how thyroid levels change with aging and whether these changes are associated with true thyroid disease and/or adverse health outcomes.

THE FULL ARTICLE TITLE
Xu Y, et al. Thyroid Studies Collaboration. Natural history of thyroid function in ageing: an individual participant data analysis of 137 488 participants from 31 prospective cohort studies. Lancet Diabetes Endocrinol 2026;14(6):485-497.

SUMMARY OF THE STUDY
Researchers evaulated results from 137,488 adults across 31 population-based groups in Europe and the United States that included repeated measurements of TSH and free T4 (FT4). Individuals with known thyroid disease, those taking thyroid medications, and pregnant individuals were excluded.

The investigators examined both data comparing people of different ages at a single point in time and data tracking changes within the same individuals over time. They also studied the relationship between changes in thyroid function and death.

Overall, thyroid function remained stable with aging. Most individuals showed little change in their TSH or FT4 levels over time. Although TSH levels tended to rise with age, particularly after age 65, the increase was small. Across an entire lifetime, average TSH levels increased by less than 1 mIU/L—about 0.6 to 1.0 mIU/L depending on sex and iodine intake—numbers that represent only minor changes. In other words, aging was associated with only subtle shifts in thyroid function, not major changes. Lifestyle factors were also important – on average, smokers had lower TSH levels, while individuals with obesity had higher TSH levels.

Importantly, any meaningful change in an individual’s TSH or FT4 over time was associated with a higher risk of death from any cause compared with people whose thyroid hormone levels remained stable.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
This study shows that most adults maintain stable thyroid function as they age, and average changes in TSH are small. The findings suggest that, while small increases in TSH may be normal, significant individual changes in thyroid hormone levels over time may signal an increased risk of adverse health outcomes. A mildly elevated TSH level should neither automatically trigger treatment nor be dismissed as a normal consequence of aging. Instead, clinicians should interpret thyroid test results in the context of the individual patient and follow thyroid hormone levels over time. Monitoring trends and looking for meaningful changes from a person’s usual pattern is often more helpful than relying on a single test result when distinguishing normal aging from true thyroid dysfunction.

— Phillip Segal, MD

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