Familial hypocalciuric hypercalcemia (FHH) is a benign syndrome with elevated levels of serum calcium, relative hypocalciuria, and non-suppressed serum parathyroid hormone (PTH) levels. FHH usually occurs by a heterozygous mutation of the calcium sensing receptor (Casr), but some FHH patients show no mutations of the Casr. We encountered a unique FHH family in which the proband and her mother had many calcium deposits on their skin. The proband was medicated with Levothyroxine for hypothyroidism due to an iodine transport defect (ITD). We searched for mutation of the Casr, but found none. The only change distinguishing the proband and her mother from her father was at codon 990, reported to be a polymorphic site. We investigated the frequency of polymorphism at codon 990, but a significant relationship between the three genotypes and the serum calcium concentration was excluded. At the other polymorphic sites at codon 536, 926, 986, and 1011, polymorphisms were rare in Japanese, and we could not confirm a significant relationship. In conclusion, mutation in the Casr gene alone does not explain all cases of FHH. The other mechanisms should be identified.