By using a model of musculoskeletal illness, “Najdi lambs”, sera were examined for determining the levels of the total serum vitamin D3, Ca (calcium), P (phosphorus), PTH (parathyroid hormone), ALP (alkaline phosphatase), and ALT (Alanine aminotransferase).The results showed that there was a sharp down-regulation (17.7 ± 1.07 Ng/ML) of total serum vitamin D3 in Najdi lambs that had signs of musculoskeletal disorders, poor body and hair condition scores, decrease appetite, and poor growth when compared to high levels (89.75 ± 7.06 Ng/ML) in control healthy Najdi lambs. The gene discussed is PTH; the disease is musculoskeletal system disorder.