Data on redox homeostasis and matrix metalloproteinase (MMP) activity in parathyroid adenoma are limited, although chronic inflammation and oxidative stress may contribute to the pathophysiology of primary hyperparathyroidism. In this prospective pre–post study, we assessed systemic redox status, nitrosative stress markers, and circulating MMP activity in 52 patients with parathyroid adenoma before and six months after parathyroidectomy and in 52 age- and sex-matched controls. Before surgery, patients had lower total antioxidant capacity (TAC), and higher total oxidant status (TOS) and oxidative stress index (OSI) compared to the controls. Six months after surgery, TAC returned to control levels, whereas TOS and OSI remained elevated. Nitric oxide (NO) levels decreased after parathyroidectomy, and peroxynitrite (ONOO⁻) levels at follow-up were higher than in controls. Postoperatively, the plasma activity of all six investigated MMPs (MMP-1, -2, -7, -9, -11, and -13) was lower than in the controls, whereas significant pre- to postoperative decreases were observed only for MMP-7 and MMP-11. Thus, six months after parathyroidectomy, several systemic redox, nitrosative, and MMP parameters still differed from those in the controls. Longer follow-up is required to determine whether these abnormalities normalize and to establish their clinical significance.