An Unusual Mimic of Axial Spondyloarthritis: A Case Report

Axial spondyloarthritis (SpA) is a chronic inflammatory condition predominantly affecting the sacroiliac joints and spine, typically presenting before the age of 45 years with inflammatory back pain. However, diagnostic challenges arise when atypical features and negative autoimmune markers obscure the clinical picture. We present a case of a male in his 40s with no significant medical history, presenting with a three-month history of inflammatory back pain. Despite negative human leukocyte antigen B27 (HLA-B27) status, clinical examination, including positive findings on the FABER (flexion, abduction, and external rotation) test and exaggerated muscle tenderness, raised suspicion of axial SpA. An MRI of the pelvis confirmed bilateral symmetrical sacroiliitis, supporting the diagnosis. Unexpectedly, further investigations revealed a very low vitamin D level, normal calcium levels, and elevated parathyroid hormone (PTH), suggesting secondary hyperparathyroidism. A subsequent PET scan disclosed increased uptake posterior to the right lobe of the thyroid, prompting consideration of secondary hyperparathyroidism due to severe vitamin D deficiency. Treatment with vitamin D supplementation and nonsteroidal anti-inflammatory drugs yielded remarkable improvement in symptoms, with normal repeat blood investigations post-treatment. This case underscores the importance of a comprehensive diagnostic approach in patients with inflammatory back pain, especially when classical markers such as HLA-B27 are negative. It highlights the potential interplay between axial SpA and secondary hyperparathyroidism, emphasizing the need for vigilance and interdisciplinary collaboration in clinical practice.