In order to elucidate whether or not the 'coupling defect' observed in thyroids from diabetic rats is due to a structural defect of intrathyroidal thyroglobulin (Tg), the sedimentation pattern and the stability of the thyroidal soluble iodoproteins were studied in control (C), food restricted (FR), diabetic (D) and insulin-treated diabetic (D+I) rats fed a low iodine diet either with (NID) or without (LID) iodide supplementation and labelled with 125I: acutely, 24 h prior to sacrifice and chronically, by feeding the corresponding diet labelled for 60 days. Diabetes resulted in a decrease of thyroidal weight, an increase of both thyroidal 127I content and concentration and decreased plasma TSH, irrespective of the diet. Insulin treatment reversed these alterations. Food restriction led to an intermediate situation between C and D. The iodoamino acid distribution in the acutely labelled thyroidal soluble iodoproteins showed a significant increase in the percent of organified 125I found as iodotyrosines (MIT and DIT) and a decrease of that found as iodothyronines (T3 and T4) both in D and FR. However, in the isotopically equilibrated groups, no differences were found except in LID-D where a slight increase in DIT and a decrease in T3 was found as compared to the corresponding control. The sedimentation patterns of both acutely and chronically labelled thyroidal soluble iodoproteins from all experimental groups displayed two peaks. The main one, corresponding to Tg, had a slightly lower sedimentation coefficient than the 19 S internal marker run in parallel, while the second one, relatively small, formed probably by dissociation of the main Tg peak, sedimented more slowly (12- 14 S).(ABSTRACT TRUNCATED AT 250 WORDS)