The relative distribution of thyroxine, triiodothyronine and 3,3',5'-(reverse)-triiodothyronine in various fractions of thyroglobulin

Thyroglobulin fractions rich and poor in new thyroglobulin were separated by means of DEAE-cellulose chromatography of dog thyroid extracts and by zonal ultracentrifugation in a sucrose gradient of guinea pig thyroid extract incubated at low temperature. The distrubtion of thyroxine, triiodothyronine and 3,3',5'-(reverse)-triidothyronine in hydrolysates of the different fractions was estimated by radioimmunoassays. Following DEAE-cellulose chromatography there was a small but statistically significant increase in T4/T3 ratio in thyroglobulin fractions eluted at high ionic strength--that is fractions relatively rich in stable iodine but poor in fresh thyroglobulin. There was no differences in the T4/rT3 ratios between the different fractions. The ratios between iodothyronines were almost identical in the various thyroglobulin fractions following zonal ultracentrifugation in a sucrose gradient of cold treated guinea pig thyroid extract. These findings lend no support to the possibility that a relatively high content of triiodothyronines in freshly synthesized thyroglobulin modulates the thyroid secretion towards a preferential secretion of triiodothyronine and 3,3',5'-(reverse)-triidothyronine at the expense of the secretion of thyroxine.