We have observed that membranes isolated from rat thyroids contain receptors for the insulin-like growth factors (IGF). As IGFs are known to be important mediators of tissue growth, we conducted this study to determine whether modulation of thyroid IGF receptors might be involved in TSH-stimulated hyperplasia. A substantial increase in both the weight of the thyroid and its DNA content was observed within 2 days of exposing adult male rats to 0.1% propylthiouracil (PTU) in their drinking water. Serum T4 reached unmeasurable levels and serum TSH rose 3-fold over control by the tenth day of treatment. [125I]Iodo-human(h)IGF-II binding to membranes isolated from hyperplastic glands was significantly higher than control beginning at 2 days. A maximum was reached after 5 days (13.3 +/- 0.8%/25 micrograms protein vs. a control level of 6.7 +/- 0.7%, mean +/- SEM). The increase had disappeared by 15 days of PTU exposure, paralleling the drastic fall in the growth rate of the glands. This increase in binding was specific for the thyroid, as it was not seen in other organs. In both treated and control animals, the receptor involved was shown to be type II by preferential binding to IGF-II, lack of interaction with insulin, and molecular sizing. The observed increase in binding could be accounted for by an increase in receptor site number, the affinity remaining essentially the same. We conclude that the TSH-stimulated hyperplasia of the rat thyroid, induced by PTU, is associated with an increase in the binding sites of the type II IGF receptor. This observation raises the possibility that modulation of this receptor may play a role in the mediation of the mitogenic effect of TSH on the thyroid gland.