Cell population kinetics in dog and human adult thyroid

Cell population kinetics was studied by bromodeoxyuridine (BrdU) histochemical and 3H thymidine radioautographic labelling in dog thyroids. In-vivo labelling with BrdU and in-vitro labelling of incubated slices with 3H thymidine gave similar results. This validates the use of in-vitro labelling of slices for the study of cell kinetics in the thyroid. In-vitro labelling of human thyroid slices demonstrated a labelling index of 13.4 x 10(-5) for follicular cells; assuming an S phase of 10 h, this corresponds to a turnover time of the order of 8.5 years for the follicular cells. Stromal cells appear to turn over faster. These results show for the first time that human thyroid cells divide about five times during adulthood and therefore that the steady state level of thyroid cell mass results from a balance between cell division and cell loss. A shorter turnover time was found as expected in the thyroid of an adolescent and in follicular colloid nodules.