SUN-352 - Personal Light Exposure Patterns and Risk of Hypothyroidism in the UK Biobank

Background:
The circadian timing system, for which the primary time cue is light, influences the hypothalamic-pituitary-thyroid (HPT) axis. Habitual light exposure that disrupts the circadian system may therefore affect thyroid disease risk, especially hypothyroidism, which is highly prevalent. However, as yet no population-level prospective studies have investigated this.

Aims:
To examine whether objectively measured personal light exposure patterns, during the day- and night-time, are associated with incident hypothyroidism.

Methods:
We analyzed 88,905 UK Biobank participants (mean [SD] age, 62.4 [7.8] years; 50,577 female [56.9%]) with one-week wrist-worn light data (Axivity AX3). Daytime (07:30 AM to 8:30 PM) and nighttime (12:30 AM to 06:00 AM) light exposures were empirically derived from 24-hour light profiles using an unsupervised factor analysis. Incident hypothyroidism was identified through linked health records across 9.5 years (June 2013 to November 2022) from UK Biobank participants who wore light sensors under free-living conditions. Cox proportional hazards models were adjusted for demographic, socioeconomic, physical activity, metabolic, sleep, and environmental factors. We excluded shift workers and participants with baseline diagnoses of thyroid dysfunction.

Results:
Individuals in the highest decile of nighttime light exposure had an increased risk of developing hypothyroidism, relative to the lowest 50% (Hazard Ratio (HR) = 1.25, 95% Confidence Interval (95% CI) [1.06–1.48], P = 0.007. The same observation was also present once adjusted for sleep efficiency: HR 1.25 [1.06 – 1.49], P=0.007, or sleep timing: HR 1.25 [1.05–1.47], P = 0.011. Daytime light exposure was not associated with higher hypothyroidism risk.

Conclusion:
Disrupted light exposure, especially increased light at night, was associated with incident hypothyroidism, suggesting sensitivity of the HPT axis to light exposure known to disrupt the circadian system. The strength of the current study is that these measurements are at the individual level. These results point to light at night as a potentially modifiable risk factor for hypothyroidism and support the importance of considering light as an endocrine disease modulator in future research.

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