Terazosin and Parkinson's Disease Extension Study

Inclusion Criteria: * Enrolled in the study "The Effect of alpha1- adrenergic receptor antagonist Therapy on Cardiac and Striatal Transporter Uptake in Pre-Motor and Symptomatic Parkinson's Disease" (STUDY #000540) * Capacity to give informed consent Exclusion Criteria: * Secondary Parkinsonism, including tardive * Concurrent dementia defined by a score lower than 22 on The Montreal Cognitive Assessment (MoCA) * Concurrent severe depression defined by a Beck Depression Inventory-Fast Screen (BDI fast screen) score greater than 13 * Comorbidities related to sympathetic nervous system (SNS) hyperactivity * Heart failure (LVEF \<45%) * Recent myocardial revascularization (\<12 weeks) * Hypertension (systolic blood pressure SBP\>150mmHg (millimeters of mercury) or diastolic blood pressure DBP\>100mmHg) * Chronic Atrial fibrillation * Concurrent Use of Beta-adrenergic antagonist * Diabetes mellitus * Chronic Obstructive Pulmonary Disease (COPD) * Untreated Sever Sleep Apnea; Apnea-Hypopnea Index (AHI) \> 30/h. * Severely reduced kidney function (Glomerular Filtration Rate\<30ml/min) * Contraindications to the use of terazosin * Recent myocardial infarction (\<48 h) * Ongoing angina pectoris * Cardiogenic shock or prolonged hypotension * Breast-feeding * Current use of phosphodiesterase type 5 inhibitors: sildenafil (ViagraTM), tadalafil (CialisTM), or vardenafil (LevitraTM) * History of priapism (persistent and painful erection) * Neurogenic orthostatic hypotension defined as symptomatic decrease in BP \> 20mmHg systolic or \> 10mmHg diastolic and HR increase \< 20bpm on supine to sitting or standing. * Blood pressure less than 110 mm Hg systolic at screening or baseline visit * Use of investigational drugs within 30 days before screening * For female participant, pregnancy, or plans for child-bearing during study period * Allergy/hypersensitivity to iodine or study medication