Autonomous growth, but not autonomous function, in embryonic human thyroids: a clue to understanding autonomous goiter growth?

Thyroid glands from six 8- to 10-week-old fetuses obtained at the time of legal abortion were cryopreserved in liquid nitrogen and transplanted into nude nu/nu mice. Histological and autoradiographic studies of the grafts labeled with [3H]thymidine and [125I]iodine showed proliferation and functional differentiation of the fetal thyroid tissue. Despite T4-mediated suppression of host TSH secretion, up to 36% of the follicular cell nuclei incorporated the thymidine label, reflecting autonomous proliferation, while iodine organification was almost entirely obliterated. Methimazole-induced TSH hypersecretion readily stimulated both growth and function of the transplanted tissue. Thus, during early development, the human thyroid largely depends on TSH for function, but not for growth. Similar findings were obtained in newborn mice, in whom 58% of the thyroid follicular cells proliferated autonomously, i.e. in the absence of TSH. The number of autonomously proliferating cells gradually declined with increasing age to about 1% in 60-day-old animals and, as reported previously, in xenotransplanted normal human thyroid tissue, whereas the number of autonomously proliferating cells was previously found to be several times higher in xenotransplanted human multinodular goiters. We, therefore, hypothesize that the rapidly and autonomously replicating cells that initiate nodule formation in human multinodular goiters reflect the persistence in the adult gland of cells with fetal growth potential.