The Effects of Recombinant TSH on Bone Turnover Markers and Serum Osteoprotegerin and RANKL Levels Giuseppe Martini, 1 Luigi Gennari, 1 Vincenzo De Paola, 1 Tania Pilli, 2 Stefania Salvadori, 1 Daniela Merlotti, 1 Fabrizio Valleggi, 1 Stella Campagna, 1 Beatrice Franci, 1 Annalisa Avanzati, 1 Ranuccio Nuti, 1 and Furio Pacini 2 Objective: Recently it was found that thyrotropin (TSH) receptors are present both in osteoclast and osteoblast and that TSH can modulate bone remodeling independent of thyroid hormones. The aim of this study was, firstly, to evaluate the effects of acute administration of TSH on bone remodeling markers both in men and in women and, secondly, to evaluate if these effects are mediated by variations in serum osteoprotegerin (OPG) and receptor activator of nuclear factor-KB ligand (RANKL). Design: We studied 30 thyroidectomized patients (10 premenopausal and 10 postmenopausal women, 10 men) affected by thyroid carcinoma on L-thyroxine therapy. Eighty age- and sex-matched subjects were used as controls. A blood sample was drawn from each patient at baseline and 3 and 5 days after recombinant human TSH (rhTSH) administration, in preparation for 131 I whole body scan, to assess serum bone markers and serum OPG and RANKL levels. Main outcome: At baseline, postmenopausal women and men had significantly higher values of bone turnover markers and serum OPG compared to control subjects. In all thyroidectomized patients serum RANKL was lower than in controls. After rhTSH administration, serum N-terminal propeptide of type-I procollagen (PINP), a marker of bone formation, increased significantly in postmenopausal women, while serum RANKL significantly increased after 3 days in postmenopausal patients and men returning to baseline values at day 5. Serum OPG levels did not change significantly. Conclusions: The low serum TSH observed in thyroidectomized patients on L-thyroxine therapy is associated with an increase of bone turnover in postmenopausal women and men that is associated with an increase of OPG and a decrease of serum RANKL levels. The acute TSH administration results in an increase of PINP, an index of osteoblastic activity, associated with an increase of serum RANKL. The lack of this response in pre- menopausal women suggests an influence of estrogen status on bone reactivity to TSH. Introduction T hyroid hormones exert a key role in normal skeletal development and in the maintenance of adult bone mass (1). An increase of bone remodeling has been reported in adult thyrotoxicosis, characterized by an imbalance between bone resorption and formation, which results in net bone loss and an increased risk for osteoporotic fractures (2). Re- cently, it has been found that thyrotropin (TSH) receptors (TSHR) are present both in osteoclasts and osteoblasts and that TSH can modulate bone remodeling independent of thyroid hormones (3). Indeed, both heterozygotic and homozygotic TSHR null mice are osteopenic with evidence of enhanced osteoclast differentiation (4). The suppression of osteoclast activation by TSH could be due to an inhibition of tumor necrosis factor a (TNFa) production (4). TNFa is a cytokine that enhances osteoclast formation directly (5) and by upre- gulating the stromal cells production of receptor activator of nuclear factor-KB ligand (RANKL) other than the respon- siveness of osteoclast precursors to this factor (6–7). RANKL is a cytokine produced by osteoblasts that stimulates osteo- clast activity and inhibits osteoclast apoptosis via its receptor 1 Department of Internal Medicine, Endocrinology & Metabolism and Biochemistry, Section of Internal Medicine, University of Siena, Siena, Italy. 2 Department of Internal Medicine, Endocrinology & Metabolism and Biochemistry, Section of Endocrinology, University of Siena, Siena, Italy. THYROID Volume 18, Number 4, 2008 ª Mary Ann Liebert, Inc. DOI: 10.1089=thy.2007.0166 455