**2.2. Thyroid hormone and bone remodeling**

252 Thyroid Hormone

al, 2003).

**2.1. Thyroid hormone and bone development** 

**2. Role of thyroid hormones in bone growth and metabolism** 

Thyroid hormones are critical for the skeletal development and the bone maintenance. The thyroid hormone, 3,5,3'-triiodothyronine (T3), is responsible for major actions of thyroid hormones. T3 binds to nuclear receptors that regulate gene transcription via interaction with thyroid hormone response elements of specific genes (Sap et al., 1986; Weinberger et al., 1986; Thompson et al., 1987). Recently, non-genomic actions of T3 and T4 have been described (Cheng et al., 2010). Local tissue availability of T3 seems to be regulated by type 2 and 3 deiodinase (St Germain et al, 2009). The nuclear thyroid hormone receptors (TRs) are derived from the THRA and THRB genes coding for the TRα1 and β1-2 T3-binding isoforms, truncated isoforms ∆α1, ∆α2 and ∆β3, and a TRa2 non-T3-binding isoform of unknown function (Lazar et Chin et al, 1990; Lazar, 1993; Chassande et al, 1997; Williams, 2000; Cheng et al, 2010). Expression of TRα1 and TRβ1 was described in growth plate chondrocytes, osteoblasts, and stromal cells of bone marrow (Williams et al, 1994; Abu et al, 1997; Ballock et al, 1999; Bradley et al, 1992; Bassett et Williams, 2003; Siddiqi et al, 2002). Expression of TRα in the skeleton is higher than that of TRβ (Bookout et al, 2006; O'Shea et

Studies on animal models have brought valuable insights into role of TRs in bone development and growth. Mice lacking TRβ or TRα1 did not display abnormalities in skeletal development (Forrest et al, 1996; Wikstrom et al, 1998). On the other hand, genetic disruption of both receptors (TRα1 and TRβ) led to delayed ossification and disorders in development of epiphyseal growth plates (EGPs; Gothe et al, 1999). Pax-8−/− mice, expressing all TR isoforms, but lacking the follicular cells producing T4 and T3 in the thyroid gland, displayed more severe abnormalities in bone development than mice KO for all TRs (TRα0/0, TRβ−/−) (Flamant et al, 2002). The authors concluded that the unliganded TRs (aporeceptors) on thyroid hormone responsive genes have repressor effects during bone development. In support of this, Pax-8−/−TRα0/0, but not Pax-8−/−TRβ−/−, compound mutants presented a partial rescue of the bone phenotype (O'Shea et Williams, 2002; Flamant et al, 2002). Another study was realized employing mice invalidated for TRα. These animals were euthyroid, but displayed a growth delay with abnormal bone development and ossification (Bassett et al 2007a, 2007b; Gauthier et al, 1999, 2001; O'Shea 2003, 2005). Mice lacking all TRα isoforms presented a less severe impairment of bone development than TRα-/- mice, pointing to the role of non-T3 binding TRα isoforms (∆α1 and ∆α2) (Gauthier et al, 2001). On the other hand, mice with nonfunctional TRβ displayed augmentation in circulating thyroid hormone levels associated with dysregulation of hypothalamo-pituitary-thyroid axis. These animals had skeletal signs of hyperthyroidism, increased bone mineral deposition and acceleration of growth-plate maturation, resulting in a short adult body size (Bassett et al, 2007a; O'Shea et al, 2003). These findings suggested an increased skeletal response to T3 via TRα, which was consistent with the hypothesis that elevated circulating thyroid hormone levels in TRβ mutant mice result in an increased skeletal response to T3 via TRα (O'Shea et al, 2006). Recently, GC-1, thyroid hormone analogue targeting preferentially TRβ1 over TRα1, has

Literature evidence points to the critical importance of thyroid hormones in bone remodeling and maintenance. Adult euthyroid mice invalidated for TRα have reduced osteoclastic bone resorption and increased trabecular bone volume and mineralization (Bassett et al, 2007a, 2007b), indicating a critical role of TRα in T3 action in bone cells. On the other hand, increased osteoclastic bone resorption and severe osteoporosis were demonstrated in adult TRβ mutant mice, suggestive of thyroid hormone excess in TRαexpressing skeletal cells (Bassett et al, 2007a, 2007b; Gauthier et al, 2001; O'Shea et al, 2006).

The bone architecture and strength are maintained by a balanced process of remodeling, which involves recruitment of osteoclast and osteoblasts. T3 can induce differentiation and inhibits proliferation of osteoblastic cells. T3 was shown to promote production of IL-6, IL-8, IGF-I and its binding proteins IGFBP2-4 in bone marrow stromal cells and osteoblasts (Milne et al, 2001; Siddiqi et al, 1998), and to increase the expression of several bone-related genes, including osteocalcin, collagen type I, gelatinase B and collagenase 3 (Gouveia et al 2001; Milne et al 1998; Pereira 1999; Varga et al 1997; Williams et al 1994). T3 is implicated in chondrogenesis, angiogenesis, bone matrix formation and mineralization (Himeno et al, 2002; Makihira et al, 2003; Pereira et al, 1999). In primary cultures of growth plate chondrocytes, T3 inhibits chondrocyte clonal expansion and cell proliferation, induces hypertrophic chondrocyte differentiation and promotes cartilage matrix mineralization (Robson et al, 2000).

Furthermore, T3 is involved in local signaling pathways by stimulating osteoblast responses to IGF1-I, PTH and fibroblast growth factors. T3 is a critical regulator of the Ihh - bone morphogenetic protein (BMP) – PTHrP feedback loop (Stevens et al, 2000). Hypothyroidism is marked by increased PTHrP expression and impaired hypertrophic chondrocyte differentiation (Stevens et al, 2000). In hyperthyroidism, reduced expression of PTHrP associated with augmentation of BMP enhances hypertrophic chondrocyte differentiation (Lassova et al, 2009; Stevens et al, 2000). It has also been shown that T3 regulates terminal differentiation of growth plate chondrocytes in part through controlling cell cycle progression at the G1/S restriction point (Ballock et al, 2000). T3 mediates osteoclastic bone resorption through activation of osteoblasts, which then release receptor activator for NF-κB ligand (RANKL), a member of the TNF cytokine family. RANKL is a ligand for osteoprotegerin, a cytokine that regulates osteoclastic differentiation, and functions as a key factor for osteoclast differentiation and activation by inhibiting osteoclasts apoptosis (Allain et al, 1992; Britto et al, 1994).

Thyroid Disorders and Bone Mineral Homeostasis 255

2003; Hase et al 2006; Ma et al, 2009). Yamoah et al (2008) have recently described RANKLresponsive elements on the TNF alpha gene providing new insights into regulation of TNF transcription in osteoclast formation. The role of TSH on RANKL remains controversial since the administration of the exogenous recombinant TSH in animal models and humans has been shown to increase and in other series to decrease RANKL serum levels (Martini et al, 2008; Sampath et al, 2007; Abe et al, 2003). Role of TSH in osteoblastogenesis seems to be mediated through attenuation of Wnt and VEGF signaling (Abe et al, 2003). Enhanced osteoblastogenesis in TSHR deficiency was found to be associated with increased expression of low-density lipoprotein receptor–like protein-5 and Flk-1 proteins (Abe et al, 2003). Expression of these receptors, but not that of osteoblastic transcription factors, was inhibited by rhTSH. Altogether, these observations suggest that TSH negatively modulates bone turnover, however, further research is warranted to explain in detail the regulatory

**3. Thyroid hormones and skeletal growth in infancy and adolescence** 

In prepubertal children, the linear growth is controlled mainly by GH-IGF-I axis, with influence from glucocorticoids and thyroid hormones. Thyroid hormones were shown to play an essential role for normal onset of the childhood component of growth (Heyerdahl, 1997). Role of the GH/IGF-I axis in the regulation of thyroid gland growth has recently been demonstrated (Boas et al, 2009). During pubertal period, sex steroids are important coregulators of skeletal growth. Age related consequences of thyroid dysfunction on bone development have largely been described. Nevertheless, the exact role of thyroid hormone in the peak bone mass acquisition during childhood and early adulthood is not well understood. The same is for the gender specific action of T3 in the developing skeleton

Euthyroid status is essential for normal skeletal development and linear growth. Generalized retardation in endochondral and intramembranous ossification associated with alterations in the EGPs, such as reduced thickness, disorganized columns of chondrocytes, and impaired differentiation of hypertrophic chondrocytes, have been reported in hypothyroid status during development (Lewinson et al, 1989; Stevens et al, 2000). The clinical consequences are reduced growth and skeletal abnormalities (Allain & McGregor, 1993). Theodore Kocher was awarded the Nobel Prize in Medicine in 1909 for his description of consequences of thyroidectomy. He showed the impact of hypothyroidism on the child growth (Kocher, 1883). Hypothyroid children present with the growth retardation and disproportionately short limbs in relation to the trunk. Radiographic skeletal examination may reveal, depending on the age and onset of hypothyroidism, a delayed closure of the fontanelles, enlargement of pituitary fossa and epiphyseal dysgenesis. Reilly & Smyth have described in 1937 stippled appearance of epiphyses on X-ray films in hypothyroid children. The pathognomonic nature of these changes was later confirmed by Wilkis (Wilkis, 1941). Epiphyseal dysgenesis has been demonstrated in the ossification centers that normally ossify after the onset of the hypothyroid status. Delayed appearance of ossification centers and delayed bone age are also noted in hypothyroid children. BMD

pathways.

(Gauthier et al, 1999).

Overall, T3 seems to enhance activity of osteoblasts by various mechanisms and signaling loops. Although the effects of thyrotoxicosis in adult bone are characterized by increased bone resorption, it is not known whether T3 acts directly in osteoclasts or whether effects on osteoclasts are secondary to the actions of T3 in osteoblasts.
