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Human thyroid epithelial cells in monolayer culture were found to release radioimmunoassayable insulinlike growth factor-I (IGF-I) over a 48-h culture period in serum-free medium. In the presence of supraphysiological concentrations of TSH (1–100 mU/ml) known to be inhibitory to DNA synthesis by human thyroid cells, the release of IGF-I was found to be inhibited in six thyroid cultures studied. In only one out of the six was IGF-I release increased in the presence of physiological mitogenic concentrations of TSH (0·1–100 µU/ml). Human thyroid fibroblasts, established by long-term culture of thyroid epithelial cells under fibroblast-selective conditions, also secreted IGF-I which was unaffected by the presence of TSH at both low and high concentrations.

Using a monoclonal antibody against human IGF-I, monolayer cultures of both human thyroid epithelial cells and human thyroid fibroblasts showed positive immunocytochemical staining for IGF peptide. However, fixed sections of intact thyroid tissue only showed positive staining for IGF peptide associated with the fibrous layers surrounding the thyroid follicle, with no staining of the follicular epithelial cells.

The growth of human thyroid epithelial cells was also found to be increased by IGF-I (25–100 ng/ml) added in medium plus 1 % fetal calf serum as assessed by the incorporation of [³H]thymidine into DNA. In the presence of a monoclonal antibody to IGF-I the increase in [³H]thymidine uptake in response to IGF-I was abolished as was that seen in response to TSH.

This study indicates a possible paracrine/autocrine role of IGF-I in the regulation of human thyroid epithelial cell proliferation by interaction with TSH.

Journal of Endocrinology (1989) 123, 495–500

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