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Metabolic syndrome ups risk of subclinical hypothyroidism

Tristan Manalac
03 Aug 2017

Patients with metabolic syndrome are at a higher risk of subsequent development of subclinical hypothyroidism, a recent study from Taiwan has shown.

“In this prospective cohort study, participants with metabolic syndrome were associated with a significantly increased risk of developing subclinical hypothyroidism during an average follow-up of 4 years compared to those without metabolic syndrome at the baseline,” said researchers.

A total of 68,743 participants (mean age 41.07 years; 52.5 percent female) accomplished a self-administered questionnaire about lifestyle factors, medical history and educational level. Participants also underwent routine laboratory and biochemical tests, and anthropometric measurements.

Cox proportional hazards were used to determine the risk of incident hypothyroidism, defined as thyroid-stimulating hormone (TSH) levels >5 µU/mL and T4 <4.5 µg/dL for overt hypothyroidism or TSH >5 µU/mL and T4 between 4.5 and 12 µg/dL, in individuals with and without metabolic syndrome.

Two percent (n=1,151) of the participants had hypothyroidism, of which 92 cases were determined to be overt while 1,059 were subclinical. Criteria for metabolic syndrome were fulfilled by 16.6 percent (n=11,437). [Sci Rep 2017;7:6754]

A cross-sectional analysis of the cohort showed that the male gender (odds ratio [OR], 1.43; 95 percent CI, 1.37 to 1.49), hypercholesterolaemia (OR, 2.50; 2.37 to 2.64), hyperuricaemia (OR, 2.78; 2.67 to 2.90), age 40 years (OR, 4.74; 4.52 to 4.96), and both overt (OR, 1.89; 1.19 to 2.99) and subclinical (OR, 1.48; 1.28 to 1.71) hypothyroidism were significantly associated with a higher risk of metabolic syndrome.

A cohort of 66,822 participants was used for the longitudinal analysis after excluding patients with baseline hypothyroidism and missing information. Over an average follow-up of 4.2 years, 1,247 (51 overt and 1,196 subclinical) new cases of hypothyroidism of were reported.

The crude incidence rates of total, overt and subclinical hypothyroidism were 5.9, 0.2 and 250 per 1,000 person-years. The risks for both total (hazard ratio [HR], 1.17; 1.00 to 1.38) and subclinical (HR, 1.21; 1.03 to 1.42) hypothyroidism were higher in participants with metabolic syndrome even after adjusting for other potential risk factors.

Notably, the risk of overt hypothyroidism was not elevated in participants with metabolic syndrome either before (OR, 0.98; 0.46 to 2.08) or after (OR, 0.47; 0.17 to 1.36) adjustments.

“Metabolic syndrome appears to be a risk factor for subclinical hypothyroidism,” according to the researchers. “In the context of higher cardiovascular risk associated with subclinical hypothyroidism and the metabolic syndrome, the current analyses suggests that thyroid dysfunction may be one intermediate factor between metabolic syndrome and cardiovascular disease.”

Its longitudinal design, large sample size and control for potential risk factors and confounders are the strengths of the present study and serve to minimize the risk of statistical bias.

On the other hand, the definitions employed in the study and the particular characteristics of the cohort may prevent it from being applied in the general population or to special cases whose metabolic syndrome may deviate from the definitions used in this study.

Moreover, despite its longitudinal design, the follow-up duration may not have been sufficiently long enough to establish the full clinical significance of the association between subclinical hypothyroidism and metabolic syndrome.

“The mechanisms and clinical consequences of the observed relations among metabolic risk factors and thyroid hypofunction needed to be explored in the future studies,” researchers said.

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