Passage of thyroid hormone into milk in rabbits

In lactating rabbits of New-Zealand breed the passage of iodothyronines from blood to milk was studied. The iodothyronines from the milk were extracted with different solvent systems and then separated either with the aid of paper chromatography in butanol-ethanol-ammonia system or on a Sephadex G-25 fine columns with the use of alkalized ethanol. At 4 h after the administration of labelled compounds the milk/serum ratio for thyroxine (T4) was 0.3, for triiodothyronine (T3) 1.2 to 2.8 and for reverse triiodothyronine (rT3) 9.0 to 18.0. When estimated with the aid of specific radioimmunoassay, the concentration of thyroxine was about 25%, while that of T3 was about 50% and that of rT3 was about 300% of the level of the corresponding compound found in plasma.