BACKGROUND: We have found abnormal expression of ICAM-1 in thyroid follicular cells from patients with Graves disease and Hashimoto disease. In this report, we present the hormonal regulation of ICAM-1 mRNA expression and the primary structure of 5-regulatory region which is important for transcriptional regulation of ICAM-1 gene. A I.S kb fragment of the 5-regulatory sequences are identified and linked to luciferase as a reporter. METHOD: Those reporter constructs were used to evaluate the expression in response to cytokines and hormones. Deletion analysis of 1.8 kb fragment of ICAM-1 promoter in FRTL-5 cells provide the evidence for the existence of several regulatory elements of enhancer and silencer in ICAM-1 gene transcription in thyroid cells. RESULTS: ICAM-1 mRNA is easily detected by Northern analysis using total RNA from FRTL-5 cells regardless of culture conditions. The transcripts of rat ICAM-1 showed single band of 2.6 kb in length. The FRT cells which was come from early FRTL cell culture did not show ICAM-1 mRNA with usual Northern analysis, We found differential regulation of ICAM-1 RNA level in different culture condition in FRTL-5 cells, The cells maintained at 3H (no hydrocortisone, no insulin, no TSH) condition showed the highest expression level compared to 4H, 5H, or 6H medium. Hydrocortisone markedly decreased the ICAM-1 RNA and insulin partially recovered the hydrocortisone induced repression. TSH which is important in growth and function of FRTL-5 cells could independently downregulate the ICAM-1 RNA levels. Forskolin (10 mM) could mimic the action of TSH on ICAM-1 mRNA. TNF-a and interferon-y increase ICAM-1 expression in FRTL-5 thyroid cells. TSH/forskolin inhibited maximal expression of ICAM-1 by TNF-a and interferon-r. Promoter activity of the ICAM-1 gene was positively regulated by cytokines, TNF-a and IFN-r and negatively regulated by thyroid stimulating hormone. The addition of TSH and FSK caused a 50% decrease in ICAM-1 promoter activity within 24 hour. The TSH and FSK action was mapped at 175 bp and 97 bp of the start of translation. The mutant construct pCAM-175 delGAS which has no GAS sequence showed no TSH mediated suppression of promoter activity. CONCLUSION: These findings suggested that hormones and cytokines differentially regulated the ICAM-1 gene expression and TSH downregulated ICAM-1 gene transcription by inhibiting the activation of IFN-r induced transcription factors which can bind the GAS of ICAM-1 promoter.