The effects of growth factors and serum on DNA synthesis and differentiation in thyroid cells in culture

The effects of three putative growth factors and serum on [Me-3H]thymidine and Na125I incorporation into thyroid cell cultures have been examined. We found that serum and EGF could stimulate radioactively labelled thymidine incorporation into confluent cultures. However, both factors completely inhibited iodine uptake and organification at low concentrations. Insulin also stimulated [Me-3H]thymidine incorporation but had no adverse effect on thyroid differentiated function. TSH examined under the same conditions was not a growth factor but was essential to maintain differentiated functions. We conclude that thyroid growth and differentiation are not mutually exclusive processes. However, EGF and serum inhibit thyroid differentiated function at very low concentrations. Elucidation of the physiological role of these factors and their mechanism of action may lead to a greater understanding of thyroid hormone biosynthesis.