Growth-retarded (grt/grt) mice are congenitally primary hypothyroid. Our previous study indicated that thyroid-stimulating hormone (TSH) responsiveness was defective in the grt/grt thyroid gland. We now report additional studies of impaired grt/grt thyroid function. Semiquantitative RT-PCR confirmed that TSH receptor (TSHR) mRNA expression in the grt/grt thyroid was significantly decreased compared with +/+ thyroids. Scatchard analysis revealed that grt/grt and +/+ mice have only one type of TSH binding site. grt/grt thyroids had fewer TSH binding sites, although this did not apparently affect the affinity of TSH for its receptor. The present data suggest that reduced TSHR levels or defects in TSHR signaling could be one of the possible defective sites in the grt/grt thyroid gland.