Cell kinetics of thyroid epithelial cells during hyperplastic goitre involution

ABSTRACT Thyroid epithelial cell kinetics were investigated in rats when a normal iodine diet was re-established after a long period of iodine deficiency supplemented with propylthiouracil (0·15%) for the last 2 months. In the first (prelabelling) experiment, all rats were labelled with a single injection of [3H]thymidine 2 days before iodine refeeding in order to follow the fate of the prelabelled cells. In the second experiment, the pulse-labelling index at the time of killing was measured; for this purpose the rats received [3H]thymidine 1 h before death. In these two experiments autoradiography was performed on histological sections. Thyroids were excised on day 0 and then at various intervals up to day 73 of iodine refeeding. Plasma tri-iodothyronine (T3) and thyroxine (T4) were very low until day 4 and then increased to reach control values on day 30. Thyroid concentration of iodide rose to 20 times the value on day 0, remained at this high level until day 2, and then diminished on day 4 to reach the control value on day 16. Plasma TSH concentrations were very high in iodine-deficient rats and did not vary significantly until day 8, when they fell rapidly to reach the control value on day 30. Thyroid weight, raised on day 0, decreased relatively quickly until day 4, then more slowly until day 30. Total thyroid epithelial cell number, high on day 0 (30·7 ×106 cells) was constant until day 16, then decreased until day 30 at which time a plateau was reached. In the prelabelling experiment, the fraction of labelled cells was high on day – 2 and doubled after 24 h because of division of the labelled cells. On day 1 after iodine refeeding this parameter decreased significantly and continued to decrease until day 16 at which time a minimum value was obtained. The mean number of silver grains per labelled nucleus decreased very slowly from 0 until day 73. The per cent of labelled and unlabelled thyroid epithelial cells in well-defined follicles was low until day 8 and increased progressively thereafter, reaching the control value on day 45. On the other hand, the pulse-labelling index on days 0 and 1 was similar to that measured on day – 2; already high on day 0, it increased markedly on day 2 (3·30 ± 0·57%), decreased abruptly on day 4, and reached the control value on day 8 (0·07 ± 0·02%). It is suggested that (1) cell death during early hyperplastic goitre involution reactivates the thyroid epithelial cells to proliferate; this phenomenon is independent of T3, T4 and TSH but clearly depends on iodide action, and it does not play a major role in goitre involution; (2) the restructuring of the gland, i.e. follicle growth, is not only achieved by newly formed cells but also by pre-existing cells (prelabelling experiment); and (3) maintenance of a large thyroid epithelial cell population, even after a long period of iodine refeeding, is the consequence of a relatively long life-span (73 days or more) of the pre-existing cell population. J. Endocr. (1987) 114, 483–490