TSH stimulation of iodine organification in early foetal rat thyroid in vitro

Thyroid glands from 15 day-old rat foetuses were incubated in Eagle's medium containing Na 125I and supplemented, or not, with TSH for 4 or 24 hours. Electron microscopic radioautographic study shows silver grains mainly in follicular cavities only in the thyroids submitted to TSH during 24 hr. A functional differentiation must therefore take place in thyroid cells under TSH stimulation.