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The importance of the presence of iodine for the formation of iodoproteins larger than 19S thyroglobulin, i.e. 27S and larger iodoproteins (large-TG), was studied. Samples of colloid were collected in vivo by micropuncture of single rat thyroid follicles after treatment with a low iodine diet or potassium perchlorate. The protein composition of the colloid was analysed by microgel electrophoresis on polyacrylamide gels.

It was observed that the large-TG fractions were reduced by approximately 50% after treatment with a low iodine diet for 3 months. The large-TG fractions were almost absent after treatment with perchlorate for 3 weeks.

Administration of 5 or 10 µg potassium iodide to the animals on a low iodine diet resulted in a significant increase of the relative amount of the large-TG fractions in the colloid within approximately 2 h, the higher dose of KI producing a more pronounced accumulation of the large-TG fractions. Inhibition of protein synthesis by cycloheximide did not prevent the increase of large-TG proteins in animals on a low iodine diet after the administration of 5 µg KI. The administration of iodide to rats treated with potassium perchlorate did not result in the appearance of large-TG fractions.

It is concluded that the aggregation of thyroglobulin molecules occurs in the follicle lumen as a consequence of iodine administration. This formation of large-TG fractions does not require exocytosis of newly synthesized protein.