Human thyroid cells in monolayer retain the ability to secrete tri-iodothyronine in response to thyrotrophin.

Confluent monolayer cultures of human thyroid cells secreted low levels of immunoassayable tri-iodothyronine (T3) and this process could be stimulated by TSH in a concentration-dependent manner. The characteristics of the response to TSH were related to the age of the thyroid cell culture both in terms of the relative sensitivity to TSH and the quantity of T3 released. Cells which had been in culture for 2-3 days (primary cultures) secreted high levels of T3 under unstimulated and TSH-stimulated conditions with a median effective dose (ED50) for TSH of 0.030 mu. TSH/ml. However, cells which had been subcultured and consequently had been in culture for a longer period of 6-7 days secreted lower levels of T3 under basal and stimulated conditions. This was approximately 30% of that released from primary cultures with and ED50 for TSH of 0.1 mu. TSH/ml. Reorganization of human thyroid cells into follicular structures was seen during growth with TSH but these cultures showed little response to subsequent acute stimulation by TSH; the return of a diminished, less sensitive response to TSH was seen after a recovery period of 8 h. The time-course of T3 release was dependent on the TSH concentration with low TSH concentrations stimulating T3 secretion after increased incubation periods. Human thyroid cells had lost the ability to concentrate and organify free iodide after several days in culture but were still secreting T3. This indicates the presence of intracellular stores of T3 which are released on stimulation with TSH, rather than new synthesis of T3.