The Clinical Significance of Thyroid Uptake in ¹²³ I- meta-iodobenzylguanidine Scintigraphy in Parkinson’s disease

Background: The thyroid gland receives sympathetic innervation. 123I-meta-iodobenzylguanidine (123I-MIBG) is a noradrenaline analog taken up by adrenergic nerve terminals. Although thyroid uptake can be seen on 123I-MIBG scintigraphy, its quantitative value is poorly understood. This study examined the clinical relevance of thyroid 123I-MIBG uptake in Parkinson's disease (PD). Methods: A total of 233 de novo patients were enrolled in a longitudinal cohort and underwent 123I-MIBG scintigraphy. Early and late heart-to-mediastinum ratio (HMR) and early thyroid-to-mediastinum ratio (TMR) were calculated. Baseline TMR was estimated using a mixed model after verifying the empirical linear decline over time. Observed and model-implied uptake ratios were analyzed in relation to adrenergic blood pressure (BP) control, motor and non-motor burdens, and quality of life (QoL). Baseline thyroid uptake was further assessed for its influence on longitudinal disease progression. Results: Patients were assessed from early PD (mean age, 68.7 ± 8.7 years; median disease duration, 1.0 year). Mean follow-up was 57.3 ± 22.1 months. Lower baseline HMR correlated with greater orthostatic BP changes, autonomic symptom burden, and worsening motor, non-motor functions, and QoL. The TMR showed no significant correlations overall; however, it was associated with a higher occurrence of supine hypertension, orthostatic hypotension, and non-dipper or nocturnal hypertension. Thyroid uptake did not influence longitudinal changes in motor, non-motor, cognitive, and QoL measure, though higher uptake showed trends toward slower cognitive decline and better QoL. Conclusions: Thyroid 123I-MIBG uptake did not predict disease progression, but may indicate early blood pressure dysregulation, and could reflect non-motor vulnerability in PD.