Comment: does L-thyroxine prevent or cause stroke in hypothyroidism?

The association between overt hypothyroidism and atherosclerotic risk factors, especially hypertension and dyslipidemia, is clear. To date, only a few small and methodologically flawed epidemiologic studies have investigated the relationship between hypothyroidism and stroke, precluding definitive conclusions. In the current issue of Neurology®, Karch and Thomas(1) present a large well-designed case-control study investigating this relationship. They compared 34,907 patients with autoimmune thyroiditis on treatment with l-thyroxine and 149,632 matched individuals without autoimmune thyroiditis from a UK primary care electronic health record system. They found a slight increase in the risk of stroke in patients with autoimmune thyroiditis (adjusted relative risk = 1.10, 95% confidence interval: 1.01-1.20) compared with controls. Some of this increased risk of stroke was independent of cardiovascular risk factors.