Dystroglycan is a high affinity laminin-binding glycoprotein originally described as a member of the dystrophin-associated glycoprotein complex in muscle. We have demonstrated the presence of dystroglycan in the thyroid using immunocytochemistry, immunoblots, ligand binding assays, and relative quantitative RT-PCR. In intact rat thyroid glands, antibodies against the alpha (extracellular, laminin-binding subunit) and beta (cytoplasmic/membrane bound) portions of the dystroglycan protein reacted at basolateral membranes where they colocalized with laminin. Western-blotted protein from the Fischer rat thyroid cell line FRTL-5 reacted with both the alpha- and beta-dystroglycan antibodies. The alpha-dystroglycan-reactive band colocalized with laminin-binding activity, and the protein and binding activity were decreased by TSH. In contrast, in the culture medium of these cells, alpha-dystroglycan was increased by TSH. The beta-dystroglycan antibody recognized the full-length 43-kDa band and an approximately 30-kDa truncated form. The truncated form was reduced in cells cultured with TSH, whereas the full-length form was not significantly diminished by TSH. Immunofluorescence of FRTL-5 cells in the absence of TSH showed a colocalization of dystroglycan and laminin. This was disrupted by the addition of TSH and was correlated to morphological changes. PCR amplification of complementary DNA with primer pairs from alpha- and beta-dystroglycan produced appropriately sized bands, whose sequence had identical protein-coding sequences and more than 96% nucleotide homology to mouse dystroglycan sequences. Relative quantitative RT-PCR of beta-dystroglycan messenger RNA showed reduced expression in cells cultured with TSH. We conclude that dystroglycan is present in rat thyroid and in FRTL5 rat thyroid cells and that TSH reduces its expression.