Does Subclinical Hyperthyroidism Raise Fracture Risk?

Mary Chris Jaklevic

November 09, 2022

People with subclinical hyperthyroidism are at 34% greater risk of experiencing a fracture compared with those with normal thyroid function, new research shows.

The finding, from a study of nearly 11,000 middle-aged men and women followed for a median of two decades, "highlights a potential role for more aggressive screening and monitoring of patients with subclinical hyperthyroidism to prevent bone mineral disease," the researchers write.

Natalie R. Daya, MPH

Primary care physicians "should be more aware of the risks for fracture among persons with subclinical hyperthyroidism in the ambulatory setting," Natalie R. Daya, MPH, a PhD student in epidemiology at the Johns Hopkins Bloomberg School of Public Health, Baltimore, and first author of the study, told Medscape Medical News.

Daya and her colleagues published their findings on November 8 in JAMA Network Open.

Building on Earlier Findings

The results agree with previous work, including a meta-analysis of 13 prospective cohort studies of 70,289 primary White individuals with an average age of 64 years, which found that subclinical hyperthyroidism was associated with a modestly increased risk for fractures, researchers note.

"Our study extends these findings to a younger, community-based cohort that included both Black and White participants, included extensive adjustment for potential confounders, and had a longer follow-up period (median follow-up of 21 years vs 12 years)," they write.

The study included 10,946 participants in the Atherosclerosis Risk in Communities Study who were recruited in Washington County, Maryland; Forsyth County, North Carolina; Jackson, Mississippi; and the suburbs of Minneapolis.

Baseline thyroid function was measured in blood samples collected during the second visit, which occurred between 1990 and 1992. No participants in the new analysis took thyroid medications or had a history of hospitalization for fractures at baseline, and all identified as Black or White. The mean age was 57 years, 24% were Black, and 54.3% were female.

Subclinical hyperthyroidism was defined as a thyrotropin level less than 0.56 mIU/L; subclinical hypothyroidism as a thyrotropin level greater than 5.1 mIU/L; and normal thyroid function as a thyrotropin level between 0.56 and 5.1 mIU/L, with normal free thyroxine levels of 0.85-1.4 ng/dL.

The vast majority (93%) of participants had normal thyroid function, 2.6% had subclinical hyperthyroidism, and 4.4% had subclinical hypothyroidism, according to the researchers.

Median follow-up was 21 years. The researchers identified 3556 incident fractures, detected with hospitalization discharge codes through 2019 and inpatient and Medicare claims data through 2018, for a rate of 167.1 per 10,000 person-years.

Adjusted hazard ratios for fracture were 1.34 (95% CI, 1.09-1.65) for people with subclinical hyperthyroidism and 0.90 (95% CI, 0.77-1.05) for those with subclinical hypothyroidism, compared with those with normal thyroid function.

Most fractures occurred in either the hip (14.1%) or spine (13.8%), according to the researchers.

Limitations included a lack of thyroid function data during the follow-up period and lack of data on bone mineral density, the researchers write.

'An Important Risk Factor'

Endocrinologist Michael McClung, MD, founding and emeritus director of the Oregon Osteoporosis Center, Portland, who was not involved in the study, pointed out that both subclinical hypothyroidism and subclinical hyperthyroidism have been linked to greater risk for cardiovascular disease as well as fracture.

The new paper underscores that subclinical hyperthyroidism "should be included as an important risk factor" for fracture as well as cardiovascular risk, McClung told Medscape. In considering whether to treat osteoporosis, subclinical hyperthyroidism "may be enough to tip the balance in favor of pharmacological therapy," he added.

Thyroid-stimulating hormone (TSH) tests to assess thyroid function are typically ordered only if a patient has symptoms of hyperthyroidism or hypothyroidism, Daya said. Depending on the cause and severity of a low TSH level, a physician may prescribe methimazole or radioactive iodine therapy to reduce the production of thyroxine, she said.

However, well-designed studies are needed to evaluate whether treatment of subclinical thyroid dysfunction reduces the risk for fracture or cardiovascular problems and assess downsides such as side effects, costs, and psychological harm, McClung noted.

The US Preventive Services Task Force concluded in 2015 that the data were insufficient to recommend screening for thyroid dysfunction in adults without symptoms. As of a year ago, no new evidence has emerged to support an update, according to the task force's website.

"Until those studies are available, selective screening of thyroid function should be considered in all patients undergoing risk assessment for cardiovascular disease or skeletal health," McClung said.

The Atherosclerosis Risk in Communities Study has been funded by the National Heart, Lung, and Blood Institute of the National Institutes of Health (NIH) and the U.S. Department of Health and Human Services.

Daya and four other study authors reported receiving NIH grants during the study period. McClung reported no relevant financial conflicts of interest.

JAMA Netw Open. November 8, 2022. Full text

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