In mammals, leptin has been shown to interplay with thyroid hormones to modulate 5′-deiodinases in different tissues in rats
and a single injection of leptin in euthyroid rats rapidly increased liver, pituitary, and thyroid D1 activity, and simultaneously decreased brown adipose tissue and hypothalamic D2 activity which converts T4 to T3 or rT3 to 3′-T2.