This paper reviews the current understanding of thyroid hormone economy and homeostasis in elasmobranch fishes and considers those measures of the activity of the hypothalamic-pituitary gland-thyroid gland-peripheral tissue axis that are necessary for adequate assessment of thyroid hormone physiology. In particular, we focus on the value of measuring hepatic 5′-monodeiodinase (5′-MDA) activity as an indicator of the animal's cellular production rate of the active thyroid hormone, triiodo-L-thyronine (T 3 ). We also examine the characteristics of hepatic 5′-MDA activity, in vitro, in adult female dogfish (Squalus acanthias) collected from Passamaquoddy Bay, New Brunswick, Canada, and in the embryos that they were carrying. T 3 production from T 4 by hepatic homogenates in vitro was time-and temperature-dependent, and was enhanced by the presence of a thiol donor. Michaelis constant (K m ) and maximum reaction velocity (V max ) values were 3.8 × 10 -7 M and 0.29 nM T 3 /mg protein/hr, respectively. The inclusion of trimethylamine-Noxide (TMAO) or a mixture of urea, TMAO, betaine and sarcosine significantly enhanced T 3 production. Hepatic 5′-MDA activity was depressed in fish fasted for 7 days.