AbstractPrimary cultures of sheep thyroid cells have been used to study regulation of thyroid growth and function by growth factors and TSH. Cells were plated at low density to minimize contributions from the endogenously produced insulin-like growth factors and their binding proteins and other proteins or hormones secreted by thyroid cells in culture. Growth of the cells was followed for 7-11 days in medium without serum. We found that TSH by itself was unable to stimulate thyroid growth. However, the ability of insulin and IGF-I to stimulate thyroid cell growth was markedly potentiated by TSH. Thyroid function was assayed by measurement of uptake of pertechnetate and organification of iodide and also by synthesis of thyroglobulin mRNA. TSH alone was unable to stimulate thyroid function appreciably. Insulin and IGF-I were ineffective by themselves at stimulating thyroid differentiated function, but in the presence of TSH, all indices were stimulated markedly. We conclude that TSH by itself is not a growth factor for thyroid cells. However, in the presence of insulin or IGF-I, TSH potentiates the growth-stimulating properties of this hormone. Similarly, TSH by itself does not stimulate thyroid function but requires the presence of insulin or IGF-I. These data show the cooperative interactions between growth factors and TSH in regulating both thyroid growth and function.