Estrogen deficiency in postmenopausal women frequently activates osteoclasts (OC), accelerates bone resorption, and leads to osteoporosis (OP). Previous studies have demonstrated that interferon gamma (IFN gamma) could increase bone resorption and may be involved in postmenopausal OP. Fluorosis also increased the risk of fractures and dental fluorosis, and fluoride may enhance osteoclast formation and induce osteoclastic bone destruction in postmenopausal women, but the underlying mechanisms are as yet unknown. Here, we show that serum fluoride and IFN gamma levels are negatively correlated with bone mineral density (BMD) in postmenopausal women residing in a fluorotic area. Estrogen suppresses IFN gamma, which is elevated by fluoride, playing a pivotal role in triggering bone loss in estrogen-deficient conditions. In vitro, IFN gamma is inhibited by estrogen treatment and increased by fluoride in Raw264.7 cell, an osteoclast progenitor cell line. In ovariectomized (Ovx) mice, estrogen loss and IFN gamma promote OC activation and subsequent bone loss in vivo. However, IFN gamma deficiency prevents bone loss in Ovx mice even in fluoride conditions. Interestingly, fluoride fails to increase IFN gamma expression in estrogen receptor a (ER alpha)-deficient conditions, but not in ER beta-deficient conditions. These findings demonstrate that fluorosis increases the bone loss in postmenopausal OP through an IFN gamma-dependent mechanism. IFN gamma signaling activates OC and aggravates estrogen deficiency inducing OR. Thus, stimulation of IFN gamma production is a pivotal "up-stream" mechanism by which fluoride promotes bone loss. Suppression of IFN gamma levels may constitute a therapeutic approach for preventing bone loss. (C) 2016 Elsevier Inc. All rights reserved.