Mechanisms?
Caffeine is believed to modulate pituitary hormone secretion, which has been shown to influence the hypothalamic-pituitary-adrenal axis. The authors speculated that caffeine could potentially affect thyroid activity by affecting pituitary function.
“However, the effects of transient and chronic caffeine administration on human thyroid function need to be verified further, and the related mechanisms remain unclear,” they noted.
Commenting on the study, Maik Pietzner, PhD, of the Berlin Institute of Health, noted that an important limitation of the study is that various patient groups were excluded, including those with abnormal TSH levels.
“What makes me wonder is the high number of exclusions and the focus on very specific groups of people. This almost certainly introduces bias, e.g., what is specific to people not reporting coffee consumption,” Dr. Pietzner said.
Furthermore, “we already know that patients with poor metabolic health do also have slight variations in thyroid hormone levels and also have different dietary patterns,” he explained.
“So reverse confounding might occur in which the poor metabolic health is associated with both poor thyroid hormone levels and coffee consumption,” Dr. Pietzner said.
He also noted the “somewhat odd” finding that the group with the highest metabolic disorders had the lowest coffee consumption, yet the highest TSH levels.
“My guess would be that this might also be a chance finding, given that the distribution of TSH values is very skewed, which can have a strong effect in linear regression models,” Dr. Pietzner said.
In general, “the evidence generated by the study is rather weak, but there is good evidence that higher coffee consumption is linked to better metabolic health, although the exact mechanisms is not known, if indeed causal,” Dr. Pietzner added. “Prospective studies are needed to evaluate whether higher coffee consumption indeed lowers the risk for thyroid disease.”
A version of this article first appeared on Medscape.com.