Objective: This study aims to analyze (1) under which conditions quiescent stem cells derived from human goiters can be propagated to outgrow and (2) if these cells have retained the capacity to differentiate into thyroid cells.

Design: Stem cells were isolated by fluorescence-activated cell sorting (FACS) as a side population by the Hoechst 33342 efflux technique. Growth pattern of stem cells and co-cultures of stem cells with thyrocytes grown as monolayer and in Matrigel was investigated. Expression of stem cell markers, endodermal markers and thyroid specific markers were analyzed by RT-PCR. In stem cell-derived thyrocytes, embedded in collagen to form follicles, TSH-dependent ¹²⁵iodide uptake was measured.

Results: Stem cells were isolated as a side population from a non-side population fraction that consisted of endodermal marker-positive cells and thyroid cells. Intense growth stimulation of stem cells in co-culture with thyrocytes resulted in formation of non-adherent, three-dimensional spheres that consisted of highly proliferating stem cells with their characteristic expression profiles. In response to TSH and serum, sphere-derived progenitor cells differentiated into thyrocytes that expressed PAX8, thyroglobulin, sodium iodide symporter, thyroid-stimulating hormone receptor, and thyroperoxidase mRNA, and showed TSH-dependent ¹²⁵iodide uptake.

Conclusion: Quiescent stem cells derived from goiters can be propagated to form spheres that consist of highly proliferating stem cells that are able to differentiate TSH-dependently into thyroid cells. Compared to thyrocytes, stem cells display a much higher proliferation rate upon acute growth stimulation which may suggest a putative role of the offspring of stem cells in the chronic growth factor-stimulated nodular transformation of the thyroid.