[Thyroid and growth factors]

Although TSH is the major regulator of thyroid growth and function, the role of TSH in thyroid cell growth in vitro is controversial. Recent studies have shown that a number of growth factors, cytokines, vitamins, hormones and other reagents including iodide, lithium or retinoids modulate proliferation of cultured thyroid cells. Insulin-like growth factors (IGFs) stimulate growth of thyroid cells from a variety of species and also enhance expression of differentiated functions stimulated by TSH. Epidermal growth factor (EGF), fibroblast growth factor (FGF), and Transforming growth factor (TGF)-alpha are also mitogens to thyroid cells, but they are inhibitory to differentiated functions. In contrast, TGF-beta is a potent inhibitor to thyroid cell growth and functions. Indirect evidence suggests that these growth factors are involved in growth regulation of normal and neoplastic thyroid cell growth in autocrine/paracrine manner.