Glial cell line-derived neurotrophic factor family receptor (GFRα) 4, the binding receptor for persephin, is coexpressed with the signaling Ret receptor tyrosine kinase predominantly in thyroid calcitonin-producing C cells. We show by in situ hybridization and immunohistochemistry that the functional, glycolipid-anchored form of GFRα4 is produced in mouse only in the C cells but not in parathyroid gland or in the brain. C cells expressed functional GFRα4 throughout postnatal development, whereas Ret expression in these cells decreased postnatally and was undetectable in adults. To understand the physiological role of GFRα4, we produced GFRα4-deficient [knockout (KO)] mice. No differences were observed between wild-type and GFRα4-KO littermate animals in growth, gross behavior, or viability. The number and morphology of the thyroid C cells were indistinguishable between the genotypes in both newborn and adult age. However, thyroid tissue calcitonin content was reduced by 60% in newborn and by 45% in 3-wk-old GFRα4-KO mice compared with wild-type controls. In contrast, thyroid calcitonin levels were similar in adult animals. Consistent with the reduced calcitonin levels, bone formation rate in juvenile GFRα4-KO mice was increased. In conclusion, this study indicates a novel role for endogenous GFRα4 signaling in regulating calcitonin production in thyroid C cells of young mice.