Thyroid Hormone Therapy and Incident Stroke
Stroke is a leading cause of death and disability and there is a need to identify modifiable risk factors.

This study aimed to determine the relationship between thyroid hormone treatment intensity and incidence of atrial fibrillation and stroke. Retrospective cohort study using data from the Veterans Health Administration between 2004 and 2017, with a median follow-up of 59 months.

The participants were 733,208 thyroid hormone users aged more than 18 years with at least two thyroid stimulating hormone (TSH) measurements between thyroid hormone initiation and incident event or study conclusion (406,030 thyroid hormone users with at least two free T4 measurements).

-- Overall, 71,333/643,687 (11.08%) participants developed incident atrial fibrillation and 41,931/663,809 (6.32%) stroke.

-- In multivariable analyses controlling for pertinent factors such as age, sex and prior history of atrial fibrillation, low TSH or high free T4 levels and high TSH or low free T4 levels were associated with higher incidence of stroke compared to euthyroidism.

-- Risk of developing atrial fibrillation and stroke was cumulative over time for both patients with exogenous hyperthyroidism and hypothyroidism.

Conclusively, both exogenous hyper- and hypothyroidism were associated with increased risk of stroke, highlighting the importance of patient medication safety.