The effect of thyroid stimulating hormone suppressive therapy on bone geometry in the hip area of patients with differentiated thyroid carcinoma

Association between serum TSH concentration and bone mineral density: an umbrella review

INTRODUCTION

The aim of this study was to summarize the results of previous studies, standardize the data, and present new statistical results in order to provide physicians with clinically significant outcomes regarding the association between serum TSH concentration and bone mineral density (BMD).

METHODS

To perform this umbrella review, a systematic search was conducted in which major online medical databases, such as PubMed, Web of Science, Embase, Scopus, Cochrane Library, and Google Scholar, were searched for meta-analyses and systematic reviews regarding the effect of TSH on BMD. Furthermore, all primary studies were screened for statistical analysis.

RESULTS

The statistical outcomes of the present study were based on the data of 75,898 patients. The pooled risk ratio of any kind of fracture in patients with subclinical hyperthyroidism was estimated to be 1.36 (95% CI: 1.18-1.56; p < 0.001). The SMD for BMD in the distal radius in male patients receiving L-thyroxine suppression therapy was estimated to be -0.61 (95% CI: -1.10-(-0.11); p = 0.02). Furthermore, the pooled risk ratio of any fracture in patients receiving L-thyroxine suppression therapy was estimated to be 1.98 (95% CI: 0.98 - 3.98; p = 0.06). In these patients, the BMD may significantly differ from that in non-treated patients. However, the difference depends on the type of bone.

CONCLUSIONS

Our data confirmed that subclinical hyperthyroidism has a detrimental effect on bones, causing decreased BMD. Based on the obtained results, the authors suggest that a reduced TSH serum level itself may be an individual factor associated with decreased BMD and, thus, with a greater risk of bone fracture. Nevertheless, it should be noted that the effects of TSH suppression therapy differ between areas of interest for assessing BMD. Furthermore, the results have shown that this issue may, in specific areas, concern not only postmenopausal women but also male patients. These conclusions should contribute to a careful consideration of the application of TSH suppressive therapy in all patients. Particular attention should be given to patients after DTC, while all the advantages and disadvantages of implementing L-thyroxine therapy should be individually considered.

Physical activity and reduced risk of fracture in thyroid cancer patients after thyroidectomy - a nationwide cohort study

Objectives

Levothyroxine suppressive therapy following thyroidectomy for thyroid cancer patients is considered as a risk factor for osteoporosis and fragility fractures. We evaluated the association of regular exercise and exercise habit change with fracture risk in adults older than 40 years who underwent thyroidectomy for thyroid cancer.

Methods

We enrolled the patients who underwent thyroidectomy for thyroid cancer older than 40 years between 2010 and 2016 from the Korean National Health Insurance Service data, and they were followed through 2019. Based on the questionnaire of health examination within 2 years before and after surgery, whether regular exercise once a week was evaluated. The reference group for the statistical analysis was the continuing lack of physical activity group that did not exercise before or after surgery. For fractures newly diagnosed during the follow-up period, univariate and multivariate Cox regression analyses were performed for risk evaluation.

Results

We evaluated 74,774 subjects, of whom 2,924 (3.9%) experienced any fractures during a median follow-up of 4.5 years. Compared with the group consistently lack of physical activity, the group that exercised before and after surgery showed a significant decrease in the risk of any fracture, vertebral fracture, and hip fracture: adjusted hazard ratio 0.848 (95% Confidence Interval 0.771-0.932), 0.703 (0.591-0.836), and 0.405 (0.224-0.732), respectively. For vertebral fracture, a significant reduction in fracture risk was confirmed even in patients who started their regular exercise after surgery: adjusted hazard ratio 0.779 (0.648-0.936). The risk reduction for vertebral fractures upon the initiation of exercise was found to be significant in the high-risk groups of patients: women and total thyroidectomy patients.

Conclusion

We suggest that maintaining or starting regular exercise after surgery may help prevent fractures in thyroid cancer patients older than 40 years who have undergone thyroidectomy.

Evaluation and Management of Bone Health in Patients with Thyroid Diseases: A Position Statement of the Korean Thyroid Association

Thyroid hormones play an important physiological role in maintaining adult bone structure and strength. Consequently, thyroid dysfunction is related to skeletal outcomes. Overt hyperthyroidism is an established cause of high bone turnover with accelerated bone loss, leading to osteoporosis and increased fracture risk. Hyperthyroidism induced by thyroid-stimulating hormone-suppressive therapy in patients with differentiated thyroid cancer is a cause of secondary osteoporosis. In contrast, there is a lack of evidence on the negative impact of hypothyroidism on bone health. Considering the clinical updates on the importance of bone health in thyroid dysfunction, the Task Force from the Clinical Practice Guidelines Development Committee of the Korean Thyroid Association recently developed a position statement on the evaluation and management of bone health of patients with thyroid diseases, particularly focused on endogenous hyperthyroidism and thyroid-stimulating hormone-suppressive therapy-associated hyperthyroidism in patients with differentiated thyroid cancer. Herein, we review the Korean Thyroid Association's position statement on the evaluation and management of bone health associated with thyroid diseases.

The risk between thyrotropin suppression and bone mineral density in differentiated thyroid cancer

BACKGROUND

The effect of thyroid stimulating endocrine (TSH) suppression medical aid on bone mineral density (BMD) of patients with differentiated thyroid carcinoma (DTC) or differentiated thyroid malignant neoplastic disease is still controversial. Our aim was to investigate the effect of TSH suppression therapy on BMD of patients with DTC.

METHODS

A total of 1651 DTC patients with TSH-suppression medical care were analyzed by RevMan 5.3 software (https://training.cochrane.org/online-learning/core-software/revman/revman-5-download) in the present study. The PubMed and Embase databases were consistently hunted for works revealed through July 29, 2022.

RESULTS

The results indicated that a significant association between femoral bone mineral density (FN-BMD) (P = .02) or lumbar spine bone mineral density (L-BMD) (P = .04) and DTC patients with TSH-suppression therapy. However, the total hip bone mineral density (TH-BMD) was not significantly related to DTC patients with TSH-suppression therapy (P = .11). For premenopausal women, it was shown that TH-BMD (P = .02) or L-BMD (P = .01) were closely related to DTC patients with TSH-suppression therapy. However, there was no relationship between FN-BMD and DTC patients with TSH-suppression therapy (P = .06). For postmenopausal women, TH-BMD was closely related to DTC patients with TSH-suppression therapy (P = .02). It was revealed that there was no significant difference between L-BMD (P = .16) or FN-BMD (P = .26) and DTC patients with TSH-suppression therapy. For men, there was no relationship between FN-BMD (P = .94) or L-BMD (P = .29) and DTC patients with TSH-suppression therapy.

CONCLUSION

Our systematic review has demonstrated that TSH inhibition treatment mainly influence the TH-BMD or L-BMD of the DTC patients who were premenopausal women; the TH-BMD of the DTC patients who were postmenopausal women. In addition, there was no influence on the FN-BMD or L-BMD of the DTC patients who were men.

Secondary Osteoporosis

Osteoporosis is a global public health problem, with fractures contributing to significant morbidity and mortality. Although postmenopausal osteoporosis is most common, up to 30% of postmenopausal women, > 50% of premenopausal women, and between 50% and 80% of men have secondary osteoporosis. Exclusion of secondary causes is important, as treatment of such patients often commences by treating the underlying condition. These are varied but often neglected, ranging from endocrine to chronic inflammatory and genetic conditions. General screening is recommended for all patients with osteoporosis, with advanced investigations reserved for premenopausal women and men aged < 50 years, for older patients in whom classical risk factors for osteoporosis are absent, and for all patients with the lowest bone mass (Z-score ≤ -2). The response of secondary osteoporosis to conventional anti-osteoporosis therapy may be inadequate if the underlying condition is unrecognized and untreated. Bone densitometry, using dual-energy x-ray absorptiometry, may underestimate fracture risk in some chronic diseases, including glucocorticoid-induced osteoporosis, type 2 diabetes, and obesity, and may overestimate fracture risk in others (eg, Turner syndrome). FRAX and trabecular bone score may provide additional information regarding fracture risk in secondary osteoporosis, but their use is limited to adults aged ≥ 40 years and ≥ 50 years, respectively. In addition, FRAX requires adjustment in some chronic conditions, such as glucocorticoid use, type 2 diabetes, and HIV. In most conditions, evidence for antiresorptive or anabolic therapy is limited to increases in bone mass. Current osteoporosis management guidelines also neglect secondary osteoporosis and these existing evidence gaps are discussed.

Effect of TSH Suppression Therapy on Bone Mineral Density in Differentiated Thyroid Cancer: A Systematic Review and Meta-analysis

CONTEXT

Because subclinical hyperthyroidism increases the risk of osteoporosis and fractures, concerns are growing about the long-term skeletal safety of TSH suppression therapy after total thyroidectomy in patients with differentiated thyroid cancer (DTC).

OBJECTIVE

We aimed to determine the effect of TSH suppression therapy on bone mineral density (BMD) in DTC patients.

METHODS

We searched PubMed, Embase, the Cochrane library, and other sources. Eligible observational studies included DTC patients who underwent TSH suppression therapy and BMD measurement. Two independent reviewers extracted data on the studies' characteristics and outcomes and determined their risk of bias. Data were extracted from each study for postmenopausal/premenopausal women's and men's lumbar spine (LS), femoral neck (FN), and total hip (TH) BMD and summed using a random-effects meta-analysis model. The weighted mean differences with 95% CIs are expressed for the differences in outcome measurements between groups.

RESULTS

Seventeen studies (739 patients and 1085 controls) were included for quantitative analysis. In postmenopausal women, TSH suppression therapy showed a significant decrease in LS BMD (-0.03; -0.05, -0.02), and a similar trend was seen in TH. In premenopausal women, TSH suppression therapy significantly increased LS BMD (0.04; 0.02, 0.06) and FN BMD (0.02; 0.01, 0.04). In men, there was no significant association between TSH suppression therapy and BMD at any site compared with the controls.

CONCLUSION

Evidence from observational studies suggests that postmenopausal women treated with TSH suppression therapy are at risk for lower BMD. Attention should be paid to long-term skeletal safety in DTC survivors.

Effects of thyroid-stimulating hormone suppression after thyroidectomy for thyroid cancer on bone mineral density in postmenopausal women: a systematic review and meta-analysis

OBJECTIVES

We assessed thyroid-stimulating hormone (TSH) suppression effects on bone mineral density (BMD) in postmenopausal women who underwent thyroidectomy.

DATA SOURCES

PubMed, EMBASE, Cochrane Library, Web of Science and SCOPUS were searched from inception to 24 February 2021.

STUDY SELECTION

Case-control studies were included.

DATA EXTRACTION AND SYNTHESIS

Two authors independently reviewed the studies, extracted the data and performed meta-analysis of eligible studies.

RESEARCH DESIGN AND METHODS

Studies evaluating BMD in postmenopausal women with thyroid cancer who had thyroidectomy and levothyroxine therapy were included. Differences in BMD were presented as standardised mean differences (SMDs). Meta-analyses were conducted using a random-effects model.

RESULTS

Analysis of 16 case-control studies (426 patients and 701 controls without thyroid cancer) showed that stringent TSH suppression (TSH <0.10 mIU/L) after thyroidectomy had deleterious effects on the BMD of the lumbar spine in postmenopausal women compared with controls (SMD -0.55; 95% CI -0.99 to -0.10; I²=75.8%). There was no significant difference in patients with moderate TSH suppression (TSH 0.10-0.49 mIU/L). TSH suppression in postmenopausal women was not significantly associated with lower femoral neck BMD. Subgroup analysis of the lumbar spine showed that the association between stringent TSH suppression and lower BMD was consistent among studies with >10 years of follow-up (SMD -0.32; 95% CI -0.50 to -0.14). Subgroup analysis of the femoral neck showed that total thyroidectomy was related to detrimental effects on the BMD of the femoral neck (SMD -0.60; 95% CI -0.89 to -0.31; I²=90.4%), but near-total thyroidectomy was not (SMD 0.00; 95% CI -0.30 to 0.30; I²=55.6%).

CONCLUSIONS

Stringent TSH suppression had deleterious effects on the BMD of the lumbar spine after thyroidectomy in postmenopausal women. Further studies are needed to determine whether stringent TSH suppression after thyroidectomy increases the fracture risk.

Effects of royal jelly on bone metabolism in postmenopausal women: a randomized, controlled study

OBJECTIVES

This study aimed to evaluate the effects of royal jelly (RJ) supplementation on bone metabolism in postmenopausal women.

METHODS

This was a randomized, double-blind, placebo-controlled clinical trial. Seventy-two healthy postmenopausal women aged 45-60 years within 5 years after menopause were randomized into two groups: women in the RJ group (n = 36) received capsules containing dried RJ (equivalent to 3000 mg of fresh RJ); and women in the placebo group (n = 36) received placebo daily for 6 months. Bone mineral density (BMD) of the lumbar spine (L2-L4) and left proximal femur, hip structural analysis (HSA) of the left hip, and bone turnover markers were measured.

RESULTS

Although women in the placebo group experienced a significant loss of BMD and deterioration in HSA parameters of the femur, no significant differences were found in these parameters in women in the RJ group. The levels of total procollagen type 1 N-terminal propeptide (P1NP) and tartrate-resistant acid phosphatase decreased significantly in the placebo group; however, the total P1NP level, a marker of bone formation, was not significantly different in the RJ group at postintervention compared with baseline.

CONCLUSION

RJ consumption may ameliorate decreases in femoral BMD and strength in postmenopausal women.

Effect of increased levothyroxine dose on depressive mood in older adults undergoing thyroid hormone replacement therapy

OBJECTIVE

Depressive mood consequent to hypothyroidism can be reversed with levothyroxine (LT4) replacement therapy. However, it is unclear whether increasing LT4 dose confers additional mood benefits.

DESIGN AND PATIENTS

A single-blinded before-and-after study of 24 patients with hypothyroidism who were aged 65 years or older and undergoing LT4 replacement therapy with stable doses.

MEASUREMENTS

Geriatric Depression Scale (GDS-K) and Hyperthyroid Symptom Scale (HSS-K) were assessed at baseline, 3 months after increasing LT4 dose by an additional 12.5 µg/d, and finally 3 months after returning to the baseline dose.

RESULTS

Serum thyroid-stimulating hormone (TSH) concentrations decreased at the higher LT4 dose (1.95 ± 2.16 vs 0.47 ± 1.09 mIU/L, P < .001) and recovered after returning to the baseline dose. Serum-free thyroxine levels and HSS-K scores were unchanged during the study period. GDS-K scores improved on the increased dose (9.5 ± 6.6 vs 7.5 ± 4.7, P = .029), and this improvement was maintained after returning to the baseline dose (9.5 ± 6.6 vs 7.4 ± 5.4, P = .010). Higher serum TSH was independently associated with both higher GDS-K and depression risk among those with depressive mood (GDS-K > 10) at baseline.

CONCLUSIONS

Depressive mood improves with increased LT4 dose, without significant hyperthyroid symptoms or signs, in older adults undergoing thyroid hormone replacement. These findings suggest the potential for varying the treatment target for hypothyroidism based on mood status and that low-dose LT4 treatment might be an ancillary treatment for depression.

Trabecular bone deterioration in differentiated thyroid cancer: Impact of long-term TSH suppressive therapy

Background

Conflicting results has been reported regard osteoporosis and fractures in patients with Differentiated Thyroid Cancer (DTC). Our objective was to evaluate the long‐term effects of TSH suppression therapy with Levothyroxine (LT4) on trabecular bone score (TBS) and bone mineral density (BMD) in females with DTC after thyroidectomy.

Methods

About 145 women with resected DTC and receiving long‐term TSH therapy, were stratified according to the degree of TSH suppression. Mean duration of follow‐up was 12.3 ± 6.1 years. BMD and TBS, were assessed using dual‐energy X‐ray absorptiometry (DXA) and TBS iNsight (Med‐Imaps), at baseline (1‐3 months after surgery) and at the final study visit.

Results

In patients stratified by duration of TSH suppression therapy (Group I, 5‐10 years; Group II, >10 years), slight increases from baseline TSH levels were observed. Significant decreases in LS‐BMD and FN‐BMD were seen in patients after >10 years. TBS values were lower in Groups I (1.289 ± 0.122) and II (1.259 ± 0.129) compared with baseline values (P = .0001, both groups). Regarding the degree of TSH suppression, TBS was significantly reduced in those with TSH < 0.1 µU/mL (P = .0086), and not in patients with TSH suppression of 0.1.‐0.5 or >0.5 µU/mL.

Conclusions

We found deterioration of trabecular structure in patients with DTC and TSH suppression therapy below 0.1 µU/mL and after 5‐10 years of follow‐up. Significant changes in BMD according to TSH levels were not observed. Trabecular Bone Score is a useful technique for identifying thyroid cancer patients with risk of bone deterioration.

Association between Geriatric Nutrition Risk Index and Skeletal Muscle Mass Index with Bone Mineral Density in Post-Menopausal Women Who Have Undergone Total Thyroidectomy

Background: Osteoporosis is highly prevalent in postmenopausal women and may result in fractures and disabilities. Total thyroidectomy has also been associated with loss of bone mass. The aim of this cross-sectional study was to evaluate associations among nutritional status, skeletal muscle index and markers of bone turnover to bone mineral density in postmenopausal women who had undergone total thyroidectomy. Methods: Fifty postmenopausal women who had undergone total thyroidectomy were included. Body composition was measured using dual-energy X-ray absorptiometry (DXA). The Geriatric Nutritional Risk Index (GNRI) was calculated using baseline body weight and serum albumin level. Skeletal muscle mass index was calculated as the appendicular skeletal muscle mass (ASM) divided by the height squared and assessed using DXA. Results. Multivariate stepwise linear regression analysis showed that a low GNRI was significantly associated with low lumbar spine bone mineral density (BMD) and T-score, and that a low ASM/height² was significantly associated with low femoral neck BMD and T-score. A low vitamin D level was significantly associated with low femoral neck BMD and T-score and low total hip BMD and T-score. A high bone alkaline phosphatase (ALP) level was significantly associated with low femoral neck T-score and low total hip BMD and T-score. A low insulin-like growth factor-1 (IGF-1) was significantly associated with low total hip BMD and T-score. Conclusion: In the postmenopausal women who had undergone total thyroidectomy in this study, BMD was positively associated with GNRI, skeletal muscle mass index, and levels of vitamin D and serum IGF-1, and inversely associated with bone ALP level. Nutritional status, skeletal muscle mass index and bone turnover biomarkers can be used to early identify patients with a high risk of osteoporosis in this high-risk group.

Thyroid Hormone Diseases and Osteoporosis

Thyroid hormones are essential for normal skeletal development and normal bone metabolism in adults but can have detrimental effects on bone structures in states of thyroid dysfunction. Untreated severe hyperthyroidism influences the degree of bone mass and increases the probability of high bone turnover osteoporosis. Subclinical hyperthyroidism, defined as low thyrotropin (TSH) and free hormones within the reference range, is a subtler disease, often asymptomatic, and the diagnosis is incidentally made during screening exams. However, more recent data suggest that this clinical condition may affect bone metabolism resulting in decreased bone mineral density (BMD) and increased risk of fracture, particularly in postmenopausal women. The main causes of exogenous subclinical hyperthyroidism are inappropriate replacement dose of thyroxin and TSH suppressive L-thyroxine doses in the therapy of benign thyroid nodules and thyroid carcinoma. Available data similarly suggest that a long-term TSH suppressive dose of thyroxin may decrease BMD and may induce an increased risk of fracture. These effects are particularly observed in postmenopausal women but are less evident in premenopausal women. Overt hypothyroidism is known to lower bone turnover by reducing both osteoclastic bone resorption and osteoblastic activity. These changes in bone metabolism would result in an increase in bone mineralization. At the moment, there are no clear data that demonstrate any relationship between BMD in adults and hypothyroidism. Despite these clinical evidences, the cellular and molecular actions of thyroid hormones on bone structures are not complete clear.

DIAGNOSIS OF ENDOCRINE DISEASE: Evaluation of bone fragility in endocrine disorders

An underlying disease affecting bone health is present in up to 40% and 60% of osteoporotic post-menopausal women and men respectively. Among the disorders leading to a secondary form of osteoporosis, the endocrine diseases are highly represented. A frequent finding in patients affected with an endocrine-related forms of bone disease is that the skeletal fragility is partially independent of the bone density, since the fracture risk in these patients is related more to a reduction of bone quality than to a decrease of bone mass. As a consequence, bone mineral density evaluation by dual-X-ray Absorptiometry may be inadequate for establishing the risk of fracture in the setting of the endocrine-related forms of osteoporosis. In the recent years several attempts to non-invasively estimating bone quality have been done. Nowadys, some new tools are available in the clinical practice for optimizing the fracture risk estimation in patients with endocrine disorders. The aim of this review is to summarise the evidences regarding the role of the different imaging tools for evaluating bone density and bone quality in the most frequent forms of endocrine-related osteoporosis, such as obesity, diabetes, acromegaly, thyrotoxicosis, primary hyperparathyroidism, hypercortisolism and hypogonadism. For each of these disorders, data regarding both the current available tools and the future possible new techniques for assessing bone fragility in patients with endocrine diseases are reported.

Influence of Thyroid-stimulating Hormone Suppression Therapy on Bone Mineral Density in Patients with Differentiated Thyroid Cancer: A Meta-analysis

Background

The effects of subclinical hyperthyroidism on bone mineral density (BMD) induced by thyroid-stimulating hormone (TSH) suppression therapy in patients with differentiated thyroid cancer (DTC) remains unclear. We conducted a meta-analysis to determine the influence of TSH suppression therapy on BMD.

Methods

We performed a systematic search to identify studies which included BMD measurement of femoral neck, total hip or lumbar spine in patients on TSH suppression therapy for DTC. Main outcome measures were difference of BMD of femoral neck, total hip or lumbar spine measured by dual energy X-ray absorptiometry between patients and controls.

Results

A systematic search yielded a total of 11 published controlled cross-sectional studies (including about 571 patients and 836 controls). TSH suppression therapy was associated with the lower BMD of total hip (weighted mean difference [WMD], -0.023; 95% confidence interval [CI], -0.047 to 0.000; P=0.050) and spine (WMD, -0.041; 95% CI, -0.057 to -0.026; P<0.001) in postmenopausal women with DTC, while it was not associated with that in premenopausal women and men with DTC.

Conclusions

Although the included studies were limited by small numbers, results suggested possible association between chronic TSH suppression therapy and the lower BMD of spine and total hip in postmenopausal women (but not in premenopausal women and men) with DTC. A large, well-designed study with long-term follow-up would provide further insight into the influence of TSH suppression therapy and loss of BMD.

Low Thyroid Stimulating Hormone Levels Are Associated with Low Bone Mineral Density in Femoral Neck in Elderly Women

BACKGROUND AND AIMS

To determine the relationship between thyroid stimulating hormone (TSH) and bone mineral density (BMD) in elderly women.

METHODS

This is a retrospective cross-sectional population cohort study of women aged ≥65 years. All 1097 subjects had no overt thyroid dysfunction, 47 had subclinical hyperthyroidism and 100 had subclinical hypothyroidism. Overall, 167 had normal BMD, 594 had osteopenia and 336 had osteoporosis.

RESULTS

The femoral neck (FN) BMD was lower in women with lower TSH, with a high prevalance of osteoporosis and osteopenia (p = 0.036).The prevalence of osteoporosis and osteopenia was significantly low in the lowest quartile compared with the third quartile (p = 0.023) and the fourth quartile (p = 0.002), and the second low quartile, compared with the fourth quartile (p = 0.028). The differences were not significant among subclinical hyperthyroid, subclinical hypothyroid and euthyroid women. Low TSH was related to low BMDs at FN by multiple logistic regression analysis corrected for age and BMI. TSH in the lower two quartiles were independently related to osteoporosis (OR: 1.960, p = 0.023 and OR: 1.800, p = 0.037) and osteopenia (OR: 2.108, p = 0.005 and OR: 1.723, p = 0.030). Low TSH quartile (β: 0.007, p = 0.013) predicting low BMDs at FN.

CONCLUSION

Low TSH was independently related to decreased BMDs at FN in elderly women without overt thyroid dysfunction.

Role of Thyroid Hormones in Skeletal Development and Bone Maintenance

The skeleton is an exquisitely sensitive and archetypal T3-target tissue that demonstrates the critical role for thyroid hormones during development, linear growth, and adult bone turnover and maintenance. Thyrotoxicosis is an established cause of secondary osteoporosis, and abnormal thyroid hormone signaling has recently been identified as a novel risk factor for osteoarthritis. Skeletal phenotypes in genetically modified mice have faithfully reproduced genetic disorders in humans, revealing the complex physiological relationship between centrally regulated thyroid status and the peripheral actions of thyroid hormones. Studies in mutant mice also established the paradigm that T3 exerts anabolic actions during growth and catabolic effects on adult bone. Thus, the skeleton represents an ideal physiological system in which to characterize thyroid hormone transport, metabolism, and action during development and adulthood and in response to injury. Future analysis of T3 action in individual skeletal cell lineages will provide new insights into cell-specific molecular mechanisms and may ultimately identify novel therapeutic targets for chronic degenerative diseases such as osteoporosis and osteoarthritis. This review provides a comprehensive analysis of the current state of the art.