Effect of Normal Range Serum Thyrotrophin Levels on Bone Mineral Density of post-menopausal women and older men

Objective: To analyze the effect of serum TSH levels within the normal range on bone mineral density (BMD) of post-menopausal women and older men. Methods: A total of 570 subjects (305 older men aged 50 years or older and 265 post-menopausal women) with normal thyroid function were enrolled. According to the tertiles of serum normal TSH levels, men (T1, T2, and T3), and post-menopausal women (N1, N2, and N3) were divided into three groups, respectively. The differences in blood calcium, phosphorus, 25(OH) vit D, right calcaneal and left forearm BMD levels between the groups with different TSH levels were compared. Results: The right calcaneus BMD in T1 or T2 group of men was significantly lower than those in T3 group (all P<0.05). The right calcaneus and left forearm BMD in N1 or N2 group of post-menopausal women was significantly lower than those in N3 group (both P<0.05). There was no significant difference in serum calcium and phosphorus levels, 25 (OH) vit D between the three groups with different TSH levels of men or post-menopausal women (all P>0.05). The prevalence of right calcaneal osteoporosis of N1 or N2 group, and the prevalence of left forearm osteoporosis of N1 group in postmenopausal women was significantly higher than those of N3 group (all P<0.05). The TSH levels were significantly positively correlated with right calcaneal BMD of men (R=0.116, P=0.044), and positively correlated with right calcaneal and left forearm BMD of postmenopausal women (R=0.198 and R=0.135, P<0.05). BMD considered as the dependent variables, after adjusting for gender, age, BMI, waist circumference, serum calcium, phosphorus and 25 (OH) vit D levels, the risks of right calcaneal osteoporosis in N1 group of post-menopausal women were 2.278 times that of N3 The showed no association with the risks of (all Conclusions: The normal serum TSH levels of men or post-menopausal were related to BMD. The TSH levels within the lower normal range were associated with the risks of osteoporosis in post-menopausal women.

D levels, the risks of right calcaneal osteoporosis in N1 group of post-menopausal women were 2.278 times that of N3 group. The different TSH levels in men showed no association with the risks of osteoporosis (all P>0.05). Conclusions: The normal serum TSH levels of men or post-menopausal women were related to BMD. The TSH levels within the lower normal range were associated with the risks of osteoporosis in post-menopausal women.

Background
With the development of society and progression in medical technology, people paid more and more attention to the increased metabolic osteopathy. Osteoporosis is a kind of metabolic diseases that seriously harmed the health of the public, which increased the risks of fracture and had a great All the data were analyzed using SPSS 23.0 software. The normal distribution data were described as.
The comparison among multiple groups was performed by single factor analysis of variance (ANOVA) followed by LSD multiple comparsion. Chi-square test was used to analyze the counting data between multiple groups. Spearman correlation analysis was used for ranked data. After bone density stratification, binary logistic regression analysis was performed. A P value <0.05 was considered to be statistically significant.

Effect of TSH levels within the normal range on bone metabolism of older men
Comparison of parameters between the three groups with different TSH levels The total prevalence of left forearm osteoporosis in older men was 32.1% (98/305), and the total prevalence of right calcaneal osteoporosis in older men was 31.8% (97/305).
The right calcaneus BMD in the T1 or T2 group of older men was significantly lower than those in T3 group (both P<0.05). The waist circumference levels in group T1 or T2 group, and BMI levels in T2 group were significantly lower than those in T3 group (all P<0.05). There was no significant difference in age, serum calcium and phosphorus levels, 25 (OH) vit D and left forearm BMD levels between the three groups with different TSH levels of older men (all P>0.05, Table 1).

Comparison of the prevalence of osteoporosis between the three groups with different TSH levels
There was no significant difference in the prevalence of left forearm and right calcaneal osteoporosis between the three groups with different TSH levels in older men (all P>0.05, Table 2). 8 function within the normal range directly affected the bone metabolism through TSH.
The present study showed a positive correlation between the normal TSH levels of men and right calcaneal BMD, showing a significant decrease in the right calcaneal BMD in the group with TSH levels within the lower normal range (0.40-1.78) mIU/L. Till now, there were very few studies on the relationship between the TSH levels within the normal range and bone metabolism of the men. Lee et al. [5] reported that even TSH levels (0.36-1.05 mIU/L) in the lower normal range would reduce the BMD of buttocks, showing a negative impact on the geometric structures of the proximal buttocks. In addition, the osteoporosis and serum TSH levels were directly correlated, and during hypothyroidism treatment, maintaining the TSH levels within the upper limit of normal range had an important preventive effect on osteoporosis [6].
This study showed that the left forearm and right calcaneal BMD levels of N1 or N2 group in postmenopausal women were significantly lower than those of N3 group. The prevalence of right calcaneal osteoporosis of N1 or N2 group in postmenopausal women was significantly higher than those of N3 group, and the prevalence of left forearm osteoporosis of N1 group in postmenopausal women was significantly higher than that of N3 group. The normal TSH levels in postmenopausal women was positively correlated with right calcaneal and left forearm BMD. According to a previous study, a positive correlation between the TSH levels within the normal reference range and BMD of the femoral neck, hip and ward's triangle area was found in China [7]. Leader et al [2] reported that women with normal thyroid function, aged more than 65-years, and TSH levels within the lower normal range (0.35 to 1.60 mIU/L) led to decreased proximal femur BMD and alterations in the cortical bone geometry, which significantly increased the risk of hip fractures. Lee et al. [5] reported that the normal range lower TSH levels (0.36-1.17 mIU/L) also showed a negative impact on the BMD of the hip and the geometric structures of the proximal hip, resulting in decreased BMD of the femoral neck and total hip joints. All the above results suggested that TSH levels within the lower normal range may lead to decreased BMD and increased prevalence of osteoporosis in post-menopausal women.
The mechanism of correlation between the serum TSH levels and osteoporosis was still unclear. TSH receptor (TSHR) was mainly expressed in thyroid follicular cells [8], however, the studies showed that TSHR also existed in osteoblasts and osteoclasts [8-10].TSH acts as a skeletal balance spinner, affecting bone reconstruction, bone formation of osteoblasts, and bone absorption of osteoclasts by TSHR on pre-osteoblasts and osteoblasts [11]. Studies in rodents showed that TSH may prevent bone loss and stimulate bone formation [8,[12][13]. TSHR knock-out in mice with normal free T4 and T3 levels showed severe osteoporosis and also focal osteoid sclerosis, indicating the regulatory effects of TSH on bone cell formation and osteoblast differentiation [8]. The bone formation of TSHR gene knock-out mice was lower than the normal control mice, and after thyroid hormone replacement, the bone formation was increased slightly, but was still below the levels of normal mice, which indicated that TSH may have an impact on bone i.e., independent of thyroid hormone [14]. The TSH may stimulate differentiation of mouse embryonic stem cells into osteoblasts [15]. Intermittent injection of recombinant TSH can restore the osteoporosis caused by ovariectomies in adult rats and improve the skeletal strength of the rats, and this may be related to the TSHR existence on osteoblasts or osteoclasts [12].
TSH may directly affect bone reconstruction in a part of subclinical hyperthyroidism patients with lower bone health [16], and so it was considered that in addition to the traditional influence of thyroid hormone on bone, TSH may have direct effects on bone health status [17][18][19]. Hueston et al thought that TSH may be more sensitive to reflect the special relationships between thyroid function and bone metabolic indexes than thyroid hormone, which may be attributed to hypothalamus-pituitary-thyroid axis, and whether it was a positive or a negative feedback [20]. The short-term stimulation with recombinant TSH by post-menopausal women undergoing thyroidectomy can inhibit type I collagencterminal peptide and bone alkaline phosphatase, indicating that TSH can inhibit bone absorption [16].
Acar et al. [6] believed that during the treatment of hypothyroidism, maintaining TSH levels within the upper limit of reference range showed an important preventive effect on post-menopausal women's osteoporosis. In addition, the BMD was decreased with decreasing TSH levels, while the serum free thyroxine was not correlated with BMD in elderly women with normal thyroid function [5]. Therefore, TSH may have an effect on bone that was independent of thyroid hormone, and TSH levels within the higher normal range may have a protective effect on bone.
TSH can inhibit the formation, differentiation and function of osteoclasts by regulating certain cytokines [8,13]. Abe et al [8] found that TSH regulated absorption and formation of bone through low abundance G protein coupled TSHR on the osteoclast precursor, activated c-Jun amino terminal kinase (JNK) and nuclear factor kappa B (NF-κB) singaling pathway, abating the RANKL and tumor necrosis factor-alpha expression, and directly inhibited osteoclast formation. In addition, this inhibited the expression of collagen type I and osteoblasts differentiation by down-regulating low-density lipoprotein related protein 5 (LRP5) and fetal liver kinase 1 (FIK-1) expression, and inhibiting the effect of Wnt and vascular endothelial growth factor signaling pathway.

Conclusions
In conclusion, the present study indicated that the normal TSH levels of men and post-menopausal women were related to BMD. The TSH levels within the lower normal range in postmenopausal women were more likely to have osteoporosis. Therefore, the TSH levels in higher normal range may be more beneficial to prevent osteoporosis in healthy people, especially in post-menopausal women.