We have demonstrated TSH binding sites in membranes prepared from guinea pig testis. The binding of TSH to testicular homogenate was localized to the interstitial tissue and showed similar characteristics to the binding of TSH to thyroid membranes with high affinity (Ka = 2.5 × 10¹⁰ M^-1) but limited capacity (approximately 0.9–1.7 pg/mg equivalent testis homogenate). In addition, thyroid-stimulating antibodies inhibited the binding of TSH to both testicular and thyroid homogenate but failed to inhibit the binding of hCG to the testis preparation. These data suggested that the TSH-binding site was distinct from the hCG/LH binding site. Testicular and thyroid TSH-binding sites also appeared to demonstrate a similar degree of cross-reactivity with a crude preparation of hCG.

In conclusion, our findings suggest that thyroid-stimulators may influence gonadal function. Furthermore, the ability of hCG to inhibit the binding of TSH to thyroid membranes provides a possible mechanism for the known thyroid-stimulating properties of hCG.