Sexual differences in 5′-deiodinase activity in the harderian gland of Syrian hamsters and the effect of pinealectomy: Regulation by androgens

Sexual differences on thyroxine 5′‐deiodinase (5′‐D) in the Harderian gland of Syrian hamsters were investigated. We compared the 24‐h profile of 5′‐D activity in male and female hamsters, observing a clear rhythm in males but not in females. Female values were always significantly higher than male ones. After pinealectomy day/night variations in male 5′‐D activity at the time points studies were abolished, results that are in correlation with serum thyroid hormones. We also studied the regulation by androgen of the enzyme activity. Basal 5′‐D activity increased in castrated males and levels fell when animals were implanted with testosterone or its product 5α‐dihydrotestosterone (DHT). Female 5′‐D activity was also inhibited by androgens. As only the addition of DHT in the presence of epitestosterone, an inhibitor of the conversion of testosterone on DHT, in castrated males was able to decrease 5′‐D activity to control animal levels, we suggest a probable direct effect of DHT by itself. © 1996 Wiley‐Liss, Inc.