[A thyroxine-binding globulin of major biological significance in the serum of mice: demonstration and ontogenesis].

We demonstrate in the mouse serum a hitherto unrecognized major thyroxine binding globulin (TBG), analogous to human TBG or to the recently discovered rat TBG. Our demonstration is based on equilibrium dialysis, electrophoresis, immunoelectrodiffusion and autoradiography techniques. Mouse TBG displays a remarkable ontogenic pattern, with 2-3 times higher activity in foetal than in maternal serum, and a further dramatic increase after birth. Between 1 and 5 days, the T4 binding to serum reaches peak levels 7-10 times more elevated than those measured in normal or pregnant adults. We also present for the first time the ontogenesis of the thyroxine binding prealbumin (TBPA), considered until now as the only specific T4 carrier of the murine species. We show that throughout development it is the TBG, not the TBPA, which crucially governs the level of the T4-serum interactions.