There is no association between subclinical thyroid dysfunction — including subclinical hypothyroidism or hyperthyroidism — and bone mineral density (BMD) in healthy middle-aged adults. However, in postmenopausal women, subclinical hypothyroidism was found to be associated with a lower risk for osteoporosis, according to study results published in Osteoporosis International.

While overt hyperthyroidism or hypothyroidism have been linked to increased risk for bone fractures, the relationship between subclinical thyroid dysfunction and bone density and fracture risk is unclear. The goal of the current study was to explore the associations between subclinical hypothyroidism or hyperthyroidism and BMD or osteoporosis.

The Kangbuk Samsung Health Study included adults who completed a comprehensive annual or biennial examination between January 2011 and December 2016 at the Kangbuk Samsung Hospital Total Healthcare Centers in Seoul and Suwon, South Korea. The present analysis included 25,510 patients (15,761 women [mean age, 44.7 years] and 9749 men [mean age, 48.0 years]) with normal free thyroxine levels and available dual-energy x-ray absorptiometry data.

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Of 15,761 women, 14,345 (91.0%) had normal thyroid function, 1320 (8.4%) had subclinical hypothyroidism, and 96 (0.6%) had subclinical hyperthyroidism. Of 9749 men, 9148 (93.8%) had normal thyroid function, 552 (5.7%) had subclinical hypothyroidism, and 49 (0.5%) had subclinical hyperthyroidism.

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In healthy women and men, there was no apparent association between subclinical hypothyroidism or subclinical hyperthyroidism and BMD, as neither BMD nor T-score differed significantly among the subclinical hyperthyroid, subclinical hypothyroid, and euthyroid groups.

After adjustment for age, body mass index, and postmenopausal status, BMD and T-scores were statistically higher in women with subclinical hypothyroidism compared with in those with normal thyroid function. There was no similar association for men.

In postmenopausal women, there was a negative association between subclinical hypothyroidism with the risk for osteoporosis (odds ratio, 0.657; 95% CI, 0.464-0.930). This association between subclinical hypothyroidism and decreased risk for osteoporosis was not evident in men.

The researchers acknowledged several study limitations, including the cross-sectional design, inclusion of patients mainly in Korea, and possible selection bias, as only individuals with available BMD were included. Another important limitation was that most participants visited the institution only once and the definition of thyroid dysfunction was based on a single measurement of thyroid-stimulating hormone.

“[T]his study found no apparent association between subclinical hypothyroidism and subclinical hyperthyroidism…and BMD at the lumbar spine or femur in healthy middle-aged adults. Subclinical hypothyroidism was independently associated with a lower risk of osteoporosis in postmenopausal women,” concluded the researchers.

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Lee K, Lim S, Park H, et al. Subclinical thyroid dysfunction, bone mineral density, and osteoporosis in a middle-aged Korean population [published online November 13, 2019]. Osteoporos Int. doi:10.1007/s00198-019-05205-1