Abstract
BACKGROUND: Chronic, mildly insufficient sleep is associated with increased cardiometabolic risk, but whether the regulation of thyroid hormones and related growth factors are mechanisms of this association is unclear. We investigated whether 6 wk of mild sleep restriction (SR) alters levels of free thyroxine (FT4), thyroid stimulating hormone (TSH), and fibroblast growth factor-21 (FGF-21), a modulator of FT4, in adults with adequate habitual sleep (AS; 7-9 h/night). METHODS: Healthy adults participated in one of two randomized, crossover studies with identical 6-wk intervention phases: AS and SR (1.5 h/night < AS). Fasted blood samples were collected at baseline and endpoint of each phase. Outcomes were concentrations of FT4, TSH, and FGF-21 (women only). Linear mixed models tested the effects of SR vs AS on the outcomes, adjusting for baseline levels, week, sex, and sex-by-condition interaction. RESULTS: Thirty participants (20 women; 73% racial/ethnic minority; age 21-64 y [M±SD = 36.2 ± 12.8 y]) were included. In the full sample, no effects of SR on FT4 (β±SE = 0.02 ± 0.04, p = 0.654) or TSH (β±SE = -0.02 ± 0.04, p = 0.650) were observed; however, there were sex-by-condition interactions for both FT4 (p-interaction = 0.056) and TSH (p-interaction = 0.049). In sex-stratified analyses, TSH was reduced in SR vs. AS in women (β±SE = -0.11 ± 0.04, p = 0.011, Cohen's f(2) = 0.55) but not men (β±SE = 0.09 ± 0.08, p = 0.261). Among women (n = 17), FGF-21 was not significantly different between conditions (β±SE = 8.51 ± 17.70, p = 0.638). CONCLUSION: Prolonged mild SR reduces TSH in women, whereas FT4 and FGF-21 remain unaffected compared with AS. If sustained, disruptions to the thyrotropic axis in women may contribute to their more pronounced cardiometabolic risk in response to SR compared with men.