Reduced Psoas Muscle Index is Associated with Hypothyroidism in Adult Spinal Deformity

Hypothyroidism affects metabolism and muscle function, potentially contributing to sarcopenia. Its impact on psoas muscle index (PMI) in adult spinal deformity (ASD) patients remains underexplored.

To assess associations between hypothyroidism, PMI, and surgical parameters in patients undergoing corrective surgery for ASD.

A retrospective review of 235 adults undergoing ≥4-level fusion for ASD (2016-2021) was conducted. Hypothyroidism status, demographics, surgical data, and PMI at L3 and L4 were collected. Regression analyses identified predictors of hypothyroidism and PMI. TriNetX global database comparisons assessed hypothyroidism prevalence in spinal deformity versus control cohorts and across surgical complexity levels.

In the global database, hypothyroidism was more prevalent in ASD patients than in controls (14% vs. 8%, P < 0.001), with rates rising alongside arthrodesis complexity. At our institution, hypothyroid patients had significantly lower PMI at L3 (440 vs. 534 mm²/m²; P < 0.001) and L4 (637 vs. 749 mm²/m²; P < 0.001). In multivariate models, lower PMI remained independently associated with hypothyroidism (L3: OR 0.995, P = 0.001; L4: OR 0.997, P = 0.002). Hypothyroidism was also linked to fewer fused levels (OR 0.890, P = 0.040). Female sex was associated with higher hypothyroidism prevalence and lower PMI (P < 0.001).

Hypothyroidism is independently associated with lower psoas muscle mass in ASD surgery patients, indicating an association with sarcopenic muscle changes. Given its high prevalence and potential surgical relevance, thyroid dysfunction warrants greater attention in the preoperative evaluation of ASD patients.