When male rats are exposed to cold for 10 days, their bodies convert more of the inactive thyroid hormone T4 into the active form T3, leading to lower T4 and higher T3 levels in the blood.

Thyroid Hormone Levels and Expression of Genes Involved in Regulation of Thyroid System Activity in Male Rats Exposed to Prolonged Low Temperatures, and the Effect of a Thyroid-Stimulating Hormone Receptor Antagonist on These Parameters