title: Thyroid hormone levels in an egg-laying mammal, the platypus Ornithorhynchus anatinus.
abstract: Serum thyroxine (T4) and triiodothyronine (T3) levels were measured in wild-caught platypuses in southeastern Australia in summer (February) and late winter (September). There was little seasonal difference in thyroid hormone levels. Males had lower levels of both T4 and T3 than did females, but this was only statistically significant for T3. Juveniles had higher levels than adults. The levels of thyroxine ranged from 37 to 136 nmol . liter-1, and triiodothyronine from 0.3 to 1.7 nmol . liter-1. These thyroxine levels are much higher than found in marsupials or the other egg-laying mammal, the echidma, and it is suggested that they may be associated with the platypus' relatively high metabolism and growth rate.
mesh_terms: D000375:Aging; D000818:Animals; D005260:Female; D008297:Male; D009007:Monotremata / Q000097:blood*; D010987:Platypus / Q000097:blood*; D012621:Seasons; D012737:Sex Factors; D013974:Thyroxine / Q000097:blood*; D014284:Triiodothyronine / Q000097:blood*
chemical_list: D014284:Triiodothyronine; D013974:Thyroxine