The Effects of Prolonged Use of Caffeine on Thyroid and Adrenal Glands: A Retrospective Cohort Study

Abstract

Caffeine consumption has skyrocketed in recent decades as we try to match the pace with the machines. Studies have been conducted on animals and a few on humans, mainly on the acute effects of high-dose caffeine intake. Almost none have been conducted on the chronic effects of caffeine consumption. This study involved medical professionals as case subjects, who consumed caffeine daily. This study, for 3 months, involved 96 volunteers (chosen randomly w.r.t. gender and field in the medical fraternity), including people who drank >500 mg of caffeine a day and people who consumed none. People with any comorbidities at all were excluded straight away. Two sets of blood samples were drawn and assessed. Three groups were created: group 1 (>200 mg caffeine/day), group 2 (15–200 mg caffeine/day) and group 3 (<200 mg caffeine/day). The result of the study found that exposure to caffeine at doses >200 mg/day for more than 6 months leads to a significant difference in circulating free T3 ((-0.96 pmol/L ± 0.07) = (-18.5%), 95% confidence interval (CI), P = .000024) and cortisol ((-123 nmol/L ± 9.8) = (-46.8%), 95% CI, P = .00029) hormones but shows an insignificant effect on circulating thyroid-stimulating hormone (TSH) (0.4 mIU/L, 95% CI, P = .37) and adrenocorticotrophic hormone (ACTH) ((-3.2 pg/ml ± 0.3), 95% CI, P = .53) hormones, which stay within normal physiological ranges, irrespective of the daily dose of consumption. Results also highlight that women are more susceptible to a decrement in fT3 than men (relative risk = 1.58, analysis of variance (ANOVA) F-static = 7.15, P = 0.0105). Caffeine consumption in excess of 200 mg/day, for more than or equal to 6 months, causes significant derangement in basal fT3 and cortisol hormone levels, without affecting the TSH and ACTH (regulatory) hormone levels, indicating disturbance of action at the peripheral and/or cellular levels, possibly via the paraventricular nucleus (PVN)–leptin–CAR–adenosine interactions. Women are more susceptible to a decrement in fT3 levels than men (at the same dose of caffeine).