A decanucleotide tandem repeat (TR) sequence, between -162 and -140 base pairs (bp) of the minimal thyrotropin receptor promoter, decreases gene expression by repressing constitutive enhancer activity of its cAMP response element (CRE). Each decanucleotide acts additively. CRE-binding proteins and liver or thyroid nuclear extracts footprint a region including the CRE and the 3' decanucleotide, -148 to -124 bp; nuclear proteins interacting with the 3' decanucleotide protect a smaller region -148 to -135 bp. Separate groups of nuclear proteins interact with the CRE and the 3' decanucleotide; mutations of the CRE affect protein interactions with the 3' decanucleotide and the converse. Nuclear proteins bind to single- or double-stranded 3' decanucleotide DNA; those interacting with the CRE bind only double-stranded DNA. The repressor action of the 5' decanucleotide is associated with an interaction between the coding strand and a single-stranded binding protein in liver and thyroid nuclear extracts. The 5' decanucleotide is in a CT-rich region with S1 nuclease hypersensitivity, near perfect mirror images, and direct repeats. The data therefore indicate that each TR decanucleotide modulates CRE constitutive enhancer activity by different but additive mechanisms, competition versus interaction with a single-stranded binding protein, and each interacts with different nuclear proteins that are not thyroid-specific. The same region in the human thyrotropin receptor represses CRE constitutive enhancer activity by the same mechanisms, despite a nonidentical sequence and no overt TR.