KCTD family proteins typically assemble into cullin-RING E3 ligases. KCTD1 is an atypical member that functions instead as a transcriptional repressor. Mutations in KCTD1 cause developmental abnormalities and kidney fibrosis in scalp-ear-nipple syndrome. Here, we present unexpected mechanistic insights from the structure of human KCTD1. Disease-causing mutation P20S maps to an unrecognized extension of the BTB domain that contributes to both its pentameric structure and TFAP2A binding. The C-terminal domain (CTD) shares its fold and pentameric assembly with the GTP cyclohydrolase I feedback regulatory protein (GFRP) despite lacking discernible sequence similarity. Most surprisingly, the KCTD1 CTD establishes a central channel occupied by alternating sodium and iodide ions that restrict TFAP2A dissociation. The elucidation of the structure redefines the KCTD1 BTB domain fold and identifies an unexpected ion-binding site for future study of KCTD1's function in the ectoderm, neural crest, and kidney.