Thyroxine metabolism in active and torpid ground squirrels, Spermophilus richardsoni

Abstract

Sereral aspects of thyroxine (T 4) metabolism were examined in active and torpid ground squirrels, Spermophilus richardsoni. A [ 125I]T 4 turnover study showed thyroxine half-time was increased while the total distribution volume and metabolic clearance rate were decreased in hibernators compared with active animals. However, T 4 degradation rates were similar in hibernators and nonhibernators due to increased serum titres of total and, more importantly, free T 4 (FT 4) in torpid animals. Percentage FT 4 was estimated by equilibrium dialysis, and serum total T 4 concentration, by radioimmunoassay. The estimated average FT 4 values were 1.5 ng/100 ml in hibernators and 0.7 ng/100 ml in active animals (when serum was dialysed at the respective body temperatures). Serum levels of 3,5,3′-triiodo- l-thyronine (T 3) and 3,3′,5′-triiodo- l-thyronine (rT 3) were measured by radioimmunoassay. The titres of both iodothyronines were increased in dormant animals, but the ratio of rT 3 to T 3 in hibernators was less than that in active ground squirrels. These results provide further evidence for continued thyroid function during dormancy in a mammalian hibernator.