Papillary thyroid cancer (PTC) is the most common histological type of thyroid cancer (1,2) and is generally associated with a low malignancy rate and favorable prognosis; the survival rate after treatment exceeds 98% (3–7). Although the majority of patients with PTC can survive long-term, the tumor occasionally exhibits aggressive behavior, including metastasis to distant organs (8,9). However, such cases are rare, ~5% of cases of papillary thyroid carcinoma result in distant metastasis (10). Distant metastases from PTC frequently involve the lungs, bones and brain (11–13). Although bone metastases are less common compared with lung metastases, they remain a clinically important complication that can affect both quality of life and survival. Bone metastases occur in 1–7% of all PTC cases (14). Although the overall incidence of bone metastases in thyroid cancer is low, their presence can significantly influence both treatment strategies and prognosis. Bone metastases not only result in skeletal pain and functional impairment, but can also increase the risk of pathological fractures due to local bone destruction. The femur, as a load-bearing bone, is particularly vulnerable to such complications (15,16). Reports of femoral fractures due to bone metastases from thyroid cancer are rare (17) because distant metastasis of PTC is uncommon. Another reason is that the femur, as a strong and robust long bone, typically requires extensive structural compromise before a fracture occurs (18).

The present report describes the case of a 53-year-old female patient who was admitted to the hospital with left hip pain and limited mobility after a fall, which was ultimately diagnosed with an intertrochanteric fracture of the left femur, thyroid cancer with bone metastases, hyperthyroidism and lung metastases. The rarity of this case lies in the occurrence of a femoral fracture secondary to PTC metastasis. This clinical scenario not only presents challenges in diagnosis and treatment, but also offers novel insights into the metastatic behavior of thyroid cancer. Through a detailed analysis of the present case, the present report aims to enhance the understanding of bone metastases in PTC and explore the implications for patient management and prognosis.

The present report describes the case of a 53-year-old woman who was admitted to Nanxishan Hospital (Guilin, China) in August 2024 with left hip pain and limited mobility that had persisted for >16 h after a fall. The patient had no history of orthopedic trauma or orthopedic disease and had not been diagnosed with hyperthyroidism prior to admission. On admission, a physical examination revealed an externally rotated shortening deformity of the left lower limb, marked tenderness on palpation of the left hip, limitation of hip joint range of motion, positive longitudinal percussion tenderness and difficulty cooperating with a hip and knee examination due to pain.

Radiographs suggested an intertrochanteric fracture of the left femur (Fig. 1B). MRI revealed multiple abnormal signal shadows in the left proximal femur, left acetabulum, left suprapubic ramus and right femoral neck, consistent with multiple bone metastases, a pathological fracture of the left femur and local soft-tissue mass formation (Fig. 1A). CT further demonstrated a small displacement of the left femoral neck and a pathological fracture (Fig. 1B). Lung CT revealed nodules of variable sizes in both lungs (Fig. 2A), suggesting possible pulmonary metastases whereas ultrasonography showed multiple thyroid nodules [Thyroid Imaging Reporting and Data System (19) grade 5].

Laboratory test results were as follows for thyroid function: Thyroid-stimulating hormone, 0.01 uIU/ml (normal range, 0.27–4.2 uIU/ml); free thyroxine, 29.90 pmol/l (normal range, 12–22 pmol/l); free triiodothyronine, 6.2 pmol/l (normal range, 3.1–6.8 pmol/l); thyroxine, 146 nmol/l (normal range, 66–181 nmol/l); triiodothyronine, 2.17 nmol/l (normal range, 1.3–3.1 nmol/l); thyroid peroxidase antibody, 170.00 IU/ml (normal range, ≤34 IU/ml); thyroglobulin antibody, 1,578 IU/ml (normal range, 0–115 IU/ml); serum thyroglobulin, >500 ng/ml (normal range, 3.5–77 ng/ml); serum calcium, 2.01 mmol/l (normal range, 2.15–2.55 mmol/l); and alkaline phosphatase, 41 U/l (normal range, 50–135 U/l).

After comprehensive evaluation, including history taking, physical examination, imaging and laboratory review, the patient was diagnosed with the following: i) An intertrochanteric fracture of the left femur; ii) suspected thyroid cancer with bone metastases; and iii) hyperthyroidism. Given the complexity and urgency of the patient's condition, a detailed treatment plan was developed. First, treatment for hyperthyroidism (thiamazole tablets 20 mg orally once a day for 4 days) was initiated to stabilize thyroid function. Surgical intervention may be appropriate for pathological fractures caused by malignant tumors, even if survival is <6 weeks (20). Therefore, after the patient's hyperthyroid symptoms were effectively controlled, a bone tumor resection and left artificial femoral head replacement were performed 5 days post-admission. The surgery proceeded and the patient recovered uneventfully (Fig. 2B). Postoperative pathological examination was performed. Samples were fixed in 10% neutral buffered formalin at room temperature for 24 h. They were then dehydrated, embedded in paraffin, sectioned at a thickness of 4 µm, deparaffinized and rehydrated. Slices were subjected to hematoxylin and eosin staining at room temperature for 1.5 min and then observed under a light microscope. Histology confirmed PTC with metastasis to the left femoral head (Fig. 1D).

Regarding treatment of the primary thyroid cancer, an elective palliative thyroidectomy was recommended after postoperative recovery to further control disease progression. However, the patient declined the surgical intervention after being fully informed of the risks and prognosis. The patient did not consent to postoperative chemotherapy or radiotherapy and did not undergo palliative treatment for thyroid cancer. At 1 year post-surgery, follow-up through telephone revealed that the patient was in good general condition. The patient was ambulatory with the use of crutches and was able to perform normal daily activities. Further follow-up by telephone is being performed every three months.

The present report describes a rare case of an intertrochanteric fracture of the left femur caused by PTC, providing clinical insights. Although PTC typically has a favorable prognosis and is less likely to metastasize to distant organs (21) the present case illustrates that even well-differentiated PTC can metastasize aggressively, with serious consequences for the patient's quality of life and overall survival time if timely treatment is not provided. In the present case, the patient presented with a femoral fracture, where further investigation revealed metastatic PTC, underscoring the need for early and aggressive surgical management in such cases.

In addition, femoral fractures are associated with osteoporosis, particularly among older adults (22). However, the underlying cause in the present case was femoral metastasis from PTC, a rare and under-reported phenomenon. Although PTC often metastasizes to the lungs and bones, there are few reports in the literature of femoral fractures as the initial manifestation (13). The present case underscores the need to include metastatic disease in the differential diagnosis of femoral fractures, especially in patients with a history of cancer.

The co-occurrence of hyperthyroidism complicated the treatment plan in the present case. Hyperthyroidism disrupts bone metabolism and increases bone fragility, thereby raising the risk of pathological fractures (23). Therefore, in the management of these patients, controlling thyroid function to normal levels is critical for maintaining bone health.

Several studies have reported associations between thyroid cancer and femoral fractures, most frequently in the context of follicular or columnar cell variants (24–26). However, PTC presenting initially with a pathological fracture remains uncommon. A previous study revealed the molecular mechanisms underlying PTC metastasis and recurrence by comprehensively analyzing the expression profiles of microRNAs and mRNAs in PTC. It was found that the ribosomal signaling pathway and p53 signaling pathway serve key roles in bone metastasis and recurrence of PTC (27). These findings provide novel biomarkers and potential therapeutic targets for the prognostic assessment of PTC bone metastases and the development of personalized therapeutic strategies, which may hold important clinical translational value.

In conclusion, the present case not only demonstrates the rarity of pathological femoral fractures due to bone metastasis from PTC, but also highlights the need for comprehensive, multimodal evaluation in similar complex presentations to ensure accurate diagnosis and appropriate treatment planning. Patients with thyroid cancer, especially those at risk for metastasis, should undergo early screening, prompt therapeutic intervention and regular follow-up to improve both quality of life and prognosis.

Since the present patient refused palliative thyroid cancer resection and subsequent treatment, it was impossible to assess the local control rate and survival benefit of this intervention. Although this decision respects the autonomy of patient, it has objectively resulted in the following consequences: It is difficult to assess the actual benefits of palliative resection in delaying the progression of thyroid cancer (such as the potential for prolonging survival). This decision increased the complexity and risk of future treatment, underscoring the importance of comprehensive communication with patients and their families when formulating individualized treatment plans. As this is a single case report, the results cannot be extrapolated to a wider population. Although the patient retained basic functional abilities at the 1-year follow-up after surgery, the limited observation period was insufficient to reveal the tumor's long-term development.

In conclusion, the present case report highlights the fact that PTC can lead to rare intertrochanteric fractures of the femur, emphasizing the importance of early detection and timely treatment, even in cases of PTC, which is generally associated with favorable outcomes. Early and aggressive intervention should be recommended for patients with PTC to reduce the risk of metastatic spread, maintain quality of life and prevent life-threatening complications. Healthcare professionals should consider the possibility of malignant metastasis in patients presenting with atypical fractures. The present report highlights the rarity and seriousness of PTC bone metastases and the importance of early detection and treatment.