Sleep aspects and subclinical hypothyroidism: a four-year follow-up of the ELSA-Brasil study

Previous studies suggest a bidirectional relationship between thyroid dysfunction and sleep disorders. However, prospective evidence regarding the impact of sleep characteristics on subclinical hypothyroidism remains limited. This study aimed to evaluate the association between insomnia symptoms, sleep duration, and sleep debt and the incidence of subclinical hypothyroidism.

We conducted a prospective cohort analysis of 7,983 euthyroid participants from the second wave (2012-2014) of the Brazilian Longitudinal Study of Adult Health (ELSA-Brasil) who were not taking thyroid-related or psychiatric medications. Insomnia symptoms (initial, middle, and terminal), sleep duration, sleep debt, sociodemographic characteristics, and health behaviors were assessed via questionnaires. Subclinical hypothyroidism was defined as thyrotropin >4.0 μIU/mL and normal free thyroxine in the third wave (2016-2018). Crude and adjusted log-binomial regression models estimated relative risks (RR) and 95% confidence intervals (95% CI).

The incidence of subclinical hypothyroidism was 6.6% for both sexes. In women, middle insomnia was associated with a 35% reduced risk of subclinical hypothyroidism (RR: 0.65; 95% CI: 0.44-0.92). Among men, sleep debt was linked to a 30% increased incidence (RR: 1.30; 95% CI: 1.01-1.66), and in the continuous model, each additional hour of sleep debt raised the risk by 9% (RR: 1.09; 95% CI: 1.02-1.14).

Of the sleep characteristics assessed, middle insomnia due to nocturnal awakenings appeared to be protective against subclinical hypothyroidism among women, while sleep debt increased the risk among men.