It is established that thyroxine-induced ventricular hypertrophy is associated with downregulation of collagen type I gene expression and increased collagen turnover in the ventricular tissue. The present study was undertaken to test the hypothesis that circulating thyroid hormones may have a regulatory impact on the biosynthesis of the collagen matrix in the heart. To this end, we determined collagen gene expression and deposition in the hearts of male and female Sprague-Dawley rats after surgical thyroidectomy. The serum levels of 3,3'5-triiodothyronine (T3) and 3,5,3',5'-tetraiodothyronine (T4) in thyroidectomized and age/sex-matched sham-operated rats were determined by radioimmunoassay and fluorescence analysis of the serum, respectively. On day 14 post-surgery, the plasma levels of both T3 and T4 in thyroidectomized rats were decreased by greater than 85% compared with those in matching sham-operated control rats. At this time, Northern analysis of ventricular RNA from thyroidectomized rats showed a 160% (P = 0.0079) increase for pro alpha 1 (I) and a 43% increase (P = 0.0484) for pro alpha 2 (I) collagen mRNAs in the ventricular tissue of male rats compared with that in the heart of age-matched, sham-operated control rats. In the female rats, thyroidectomy led to 63% (P = 0.0469) increase in the abundance of pro alpha 1 (I) collagen and 50% (P = 0.034) increase for pro alpha 2 (I) collagen in ventricular tissue. At the protein level, the amount of collagen type I as determined by immuno-slot blotting of ventricular homogenates, was increased in the ventricular tissue of both male (131%, P = 0.0371) and female (108%, P = 0.0464) rats. Comparison of the changes in males v females showed relatively greater increases in the level of collagen type I mRNA and protein in ventricular tissue of thyroidectomized males. Of particular note, were the increases in the immunoreactive TGF-beta in ventricular tissue of thyroidectomized male and female rats which showed a pattern similar to that of changes in collagen type I. Immunofluorescent light microscopy of frozen heart sections, showed significant remodeling of the type I collagen fibers in the ventricular myocardium of thyroidectomized rats compared with age/sex-matched sham-operated rat heart. Together, these findings suggest that circulating thyroid hormones play a role in physiological regulation of collagen type I biosynthesis in the heart and this role may vary in males and females. They further suggest that normal production of collagen matrix in the heart may be dependent on the functional status of thyroid hormones.