This report described the diagnosis and treatment of hyperthyroidism in 4 (3 females and 1 male, 3–6 years of age) guinea pigs, representing 1.3% of guinea pigs presented to the university clinic over a 2.5-year period. Clinical signs included weight loss despite normal appetite and behavioral changes (eg, hyperactivity, prolonged sleep, unusual resting positions, separation from other animals). Polydipsia was reported in 2 cases. Physical examination revealed mild-to-moderate decreased nutritional status and a spherical, solid, well-defined ventral cervical mass. Additional findings in individual cases included scruffy hair coat, tachycardia (400 bpm), intensified arterial pulses, and tachypnea (140 breaths/min). Serum analysis showed elevated total thyroxine (T4) concentrations based on recently published reference intervals of 14.2–64.4 nmol/L for females and 14.2–57.9 nmol/L for males. ALT was also elevated above the upper reference limit of 61 U/L. Surgical resection was attempted in 1 case, but the patient died during anesthetic initiation. Histopathology showed papillary-cystic thyroid gland adenoma. The other 3 cases were treated with oral methimazole; clinical signs and T4 measurements were monitored. Starting methimazole doses were 1–1.4 mg/kg/day, with final dosages ranging from 2–3 mg/kg q24h and 1 case requiring 2.5 mg/kg q8h. This case was then successfully treated with radioactive iodine. All patients showed progressive weight gain and normalization of T4 levels. The patients died of unknown causes 18–28 months following initial treatment.