Background: Subclinical hypothyroidism (SCH) is postulated to increase stroke risk via atherogenic changes associated with abnormal thyroid function. However, the direct relationship of SCH with subsequent stroke is poorly studied.
Methods: In this nested case-cohort study, we prospectively evaluated the association between any SCH and severity of SCH in relation to incident ischemic stroke risk among postmenopausal women in the Women's Health Initiative Observational Study. Trained Women's Health Initiative staff, masked to thyroid status, adjudicated stroke cases. We assessed thyroid function using baseline blood specimens. Women with normal free thyroxine levels and thyrotropin (TSH) levels ≥4.69 mU/L were considered to have SCH. Primary analysis included 639 ischemic stroke cases and 2927 randomly selected subcohort members with an average of seven years of follow-up.
Results: The multivariable adjusted hazard ratios (HR) from weighted Cox models were 1.06 (95% confidence interval [CI]: 0.77, 1.46) and 0.99 (95% CI: 0.67, 1.47) for women with any SCH and with mild SCH (TSH 4.69 to 6.99 mU/L), when compared with women with normal thyroid function. The HR for moderate/severe SCH (TSH ≥7.00 mU/L) was modestly elevated (HR: 1.22; 95% CI: 0.73, 2.05).
Conclusions: We found no evidence to suggest an association between SCH and ischemic stroke among healthy postmenopausal women.