Controversy continues about whether, and to what levels of abundance, thyroid-stimulating hormone receptors (TSHR) are found in human tissues other than the thyroid gland. Restricted expression to the thyroid and orbit would suggest that TSHR represents the target autoantigen in thyroid-associated ophthalmopathy. A more generalized pattern of tissue expression would be inconsistent with TSHR acting as the autoantigen that is solely responsible for selectively targeting the immune system to the orbit. We have detected TSHR mRNA in human abdominal adipose tissue by Northern blot analysis. TSHR protein was also detected, by immunoblotting with two different antibodies, in preadipocytes isolated from human abdominal subcutaneous and omental adipose tissue and in derivative adipocytes differentiated in primary culture. Preadipocytes treated with thyroid-stimulating hormone (TSH) exhibited a sevenfold increase in the activity of p70 S6 kinase, a serine/threonine kinase recently recognized as a downstream target of TSHR in thyroid cells. Activation of p70 S6 kinase by TSH was also observed in orbital fibroblasts. Thus TSHR protein expression is found in fibroblasts from several anatomic locations, suggesting that factors other than site-limited TSHR expression must be involved in restricting the distribution of Graves' disease manifestations. Furthermore, the presence of functional TSHR in preadipocytes raises the possibility of a novel role for TSHR signaling in adipose tissue development.