Elderly men with mild thyroid dysfunction - most of whom are unaware of it - are significantly more likely to develop hip fractures, a new study reports.

Researchers found that men 65 years and older with mild thyroid dysfunction, known as subclinical hyperthyroidism or subclinical hypothyroidism, had at least a two-fold higher risk of hip fracture, which comes with potentially life-threatening complications.

“This certainly raises legitimate concern that we ought to be doing more” to mitigate the potential effects of silent thyroid dysfunction on bone, study author Dr. Jennifer Lee of the University of California, Davis, told Reuters Health. “This is an under-recognized risk factor for hip fracture.”

It’s too soon, however, to recommend that all elderly people get systematically screened for subclinical thyroid dysfunction, Lee cautioned, because doctors don’t yet know enough about the consequences of these conditions or whether they require treatment.

But these dramatic associations between thyroid dysfunction and hip fracture in elderly men suggest researchers should be looking more closely at whether relatively mild dysfunction impacts bone, and warrants treatment, she added. The findings are “substantial enough to look into this more,” Lee said in an interview.

Of particular concern, she noted, is that millions of people likely have subclinical thyroid dysfunction, and most of them don’t know it. The condition is diagnosed by measuring blood levels of thyroid-stimulating hormone (TSH).

Lower-than-average levels of TSH indicate an overactive thyroid (a condition called hyperthyroidism), and higher than average levels point to an underactive thyroid (or hypothyroidism).

The job of TSH is to tell the thyroid to make thyroxine (T4). If TSH levels go up or down but T4 levels remain normal, then patients have only very subtle (subclinical) hyperthyroidism or hypothyroidism. But once T4 levels change too, the dysfunction goes from subclinical to being more obvious, Lee explained.

When the thyroid is only “slightly out of whack,” as in subclinical dysfunction, most people don’t experience dramatic symptoms, and are therefore never diagnosed with any problems.

She estimated that 9 million people have some form of subclinical thyroid dysfunction, the risk of which increases with age.

Another problem that increases with age: hip fractures. According to the International Osteoporosis Foundation, 1.6 million people experience hip fractures every year, which is a major source of disability - afterwards, up to 1 in 5 people need long-term nursing care as a result.

To investigate whether the two conditions might be linked, Lee and her colleagues followed 3,567 people 65 and older for approximately 15 years, checking their blood for thyroid dysfunction and noting who developed fractures.

Reporting in the Archives of Internal Medicine, they found that 13 percent of elderly men had subclinical underactive thyroids (hypothyroidism) and another 2 percent had subclinical overactive thyroids (hyperthyroidism).

Ten percent of men with subclinical hypothyroidism and 14 percent of those with subclinical hyperthyroidism suffered a hip fracture, versus only 5 percent of men with normal thyroids.

Elderly women were more likely both to experience hip fractures and have subclinical thyroid dysfunction - but, in women, the risk of fracture did not increase in those with thyroid problems, and it’s not clear why, Lee noted. Both conditions are typically associated with women, she said, so it’s possible their healthcare providers had already identified and mitigated risk factors in women with thyroid problems, which helped protect them from fractures.

It’s also not clear why subclinical hypo- and hyperthyroidism would both affect bone, she added. Subclinical hyperthyroidism may simply be a milder form of overt hyperthyroidism, which is associated with bone loss. Animal studies suggest an underactive thyroid, the hallmark of hypothyroidism, affects bone turnover, which can weaken the remaining bone.

Dr. Martha Morris of Tufts University, who reviewed these findings for Reuters Health, said the potential risk of subclinical dysfunction is not “trivial,” but she agreed it’s too soon to say the thyroid problems are to blame for any fractures in men. In this group, subclinical dysfunction and hip fractures were relatively rare, so “we should not have much confidence in this study’s results for men,” Morris said.

What’s perplexing, she added, is that these findings contradict previous studies that suggested subclinical dysfunction may actually be problematic for women, and no study has yet linked hypothyroidism to hip fracture. The association here between subclinical hypothyroidism and fractures “is what we call hypothesis-generating,” Morris said in an e-mail - “i.e., we wouldn’t be too concerned unless other studies found the same thing.”

SOURCE: Archives of Internal Medicine, online November 22, 2010.