A growing literature has established the close association between tum
or progression and growth regulatory aberrations in cancer cells. Most
studies have focused on the phenomenon, that advanced tumors or metas
tases have lost the sensitivity to growth inhibitors. Metastasis are t
he primary cause of death in patients with differentiated thyroid canc
er. Proliferation and differentiation of the thyroid was supposed to b
e under the major control of only a single hormone (thyroid stimulatin
g hormone). However, we and others have shown that a complex network o
f various growth factors regulates growth and invasion of thyroid canc
er cells. Our findings highlight two aberrations of growth regulation
which may favour progression of malignancy and acquisition of metastat
ic competence: i) resistence to growth factor inhibitors and ii) growt
h autonomy of metastatic thyroid cancer cells.