Thyroid-stimulating hormone (TSH) levels decrease rapidly during the first day of prolonged physical exertion and remain suppressed throughout 5 days of combined exercise, calorie restriction, and sleep deprivation, indicating central suppression of the hypothalamic-pituitary-thyroid axis.

When the body experiences prolonged physical exertion and not enough food, the brain reduces signals to the thyroid gland to slow down metabolism and save energy. This happens because the brain detects low energy availability and directly lowers the release of a hormone that tells the thyroid to work, while also changing how the body converts thyroid hormones into their active form, making less active hormone available.