The effect of thyrotropin (TSH) levels on follicular cluster formation from grafted monodispersed rat thyroid cells.

The effect of experimental conditions which alter TSH levels on the number and size of the multifollicular thyroid clusters which develop from inoculated monodispersed thyroid cells was explored. Five week-old male Fischer rats were used as thyroid cell donors and recipients. The recipients were totally thyroidectomized (Tx) 1 week before thyroid cell transplantation (group A), hemithyroidectomized (hemi Tx) 1 week before transplantation (group B), or Tx 1 day before grafting (group C). Additional groups of recipients included intact animals maintained on low-iodine diet (LID) (group D), and on normal diet (ND) (group E). The FD50, the number of grafted cells required to produce at least one follicular cluster in 50% of the transplant sites, was the lowest in group A and the highest in group E. The TSH levels in the serum on the day of cell inoculation were highest in group A and lowest in group E. Large follicular clusters tended to be more frequent in groups A and C than in group B, but the data were variable; no large clusters were found in groups D or E. The TSH levels in groups A and C 28 days after cell implantation were higher than in group B. The FD50 is thus dependent on the concentration of circulating TSH for initiation of follicle formation; large follicular clusters result from continued elevation of TSH levels.