Endocrine Abstracts

Although the physico-chemical properties of colloid are not usually considered among the factors regulating thyroid function, it is conceivable that decreasing the viscosity of the intrafollicular colloid may facilitate diffusion of thyroglobulin (Tg) molecules within the lumina. The resulting increase in the number of contacts between Tg molecules and peroxidase at the apical membrane may promote the coupling reaction. Rats on a hormonal diet were given 0.5 μg T₄/ml of drinking water or 20 μg T₄ s.c. for 2 days up to 4 weeks to suppress endogenous TSH. 0.5 IU TSH was then injected together with 125 J. Animals were killed at intervals from 30 min up to 8 h. One thyroid lobe was used for autoradiography, the other for analyses of labeled iodoaminoacids by paperchromatography. Eight experiments with slightly varying design were performed. Within 4 h the total 125J uptake is not increased by TSH (0.22±0.03% (S.E.M.) vs 0.31±0.09% in controls). However, in rats treated with T₄ s.c. for 2 days the percentage of 125J–T₄ rises from 3.6±0.2 to 9.7±0.07% within 1 h after TSH. While in controls 80% of all follicles with a diameter >30 μm show the classical ring reaction, TSH decreases the relative fraction of ring labeled follicles to 30%. After long term preparation by T₄ given in the drinking water, the 125J–T₄ rises from 2.5±0.09 to 7.0±1.0% within 4 h after TSH. Whereas in controls, ring labeling was still present in 40% of the follicles, virtually all follicular lumina were homogeneously labeled in TSH treated animals.

Since T₄ formation is simultaneously accelerated, the enhanced contact between intraluminal Tg molecules and the membrane bound peroxidase may be the mechanism by which TSH promote early T₄ synthesis before iodine uptake is increased.

Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.

Funding: This research did not receive any specific grant from any funding agency in the public, commercial or not-for-profit sector.