Presence of free triiodothyronine and free thyroxine in thyroid follicles may be correlated with the quick secretion of thyroid hormones under certain physiological conditions.

Thyroid cells are polarized and the follicle structure, consisting of follicle epithelial cells, is a prerequisite for thyroid hormone synthesis. However, a reliable in vitro model simulating thyroid function is not currently available. To the best of our knowledge, the present study reports for the first time a simulated follicle by inoculation of human thyroid cells on the filter in a Transwell plate to maintain the polarity of thyroid cells. The iodine uptake was analyzed by arsenic and cerium catalysis spectrophotometry, as well as the secretion of free triiodothyronine (FT3) and free thyroxine (FT4) by direct chemiluminescence. The data showed that thyroid cells growing in the Transwell chamber synthesized and secreted FT3 and FT4, while the monolayer cells directly seeded in the 6-well-plate did not produce these two thyroid hormones. Regarding the iodine uptake, cells in the Transwell chamber demonstrated a markedly higher capability than the monolayer cells. The data proved that the polarity of thyroid cells could be restored using the Transwell plate, which was critical for iodine uptake and thyroid hormone synthesis. The presence of FT3 and FT4 in follicles may be correlated with the quick secretion of thyroid hormones under certain physiological conditions.