Primary hyperparathyroidism (PHPT) is a common endocrine condition. Outside of syndromic presentations, it is relatively rare in young people and during pregnancy. The diagnosis in pregnancy poses a unique challenge and is often missed as the symptoms of hypercalcemia mimic those in pregnancy. We present a case of a 20-year old woman who was found to have severe hypercalcemia after she had persistent vomiting in early pregnancy. Biochemistry showed adjusted calcium of 5.34 mmol/l (2.22.6 mmol/l), parathyroid hormone of 103.9 pmol/l (1.66.9 pmol/l) and normal Vitamin D. Further investigations revealed hypercalciuria, impaired renal function and extensive renal medullary calcification. There was no family history of hypercalcaemia and no clinical or biochemical evidence of multiple endocrine neoplasia. She was fluid resuscitated aggressively. She decided to undergo medical termination of this unplanned unwanted pregnancy at seven weeks gestation and was subsequently commenced on cinacalcet. Despite this, hypercalcaemia remained refractory. Neck ultrasound and SESTA-MIBI failed to localize a parathyroid lesion. Subsequent 4D CT and C-11 methionine PET revealed a large mediastinal mass. She had parathyroidectomy at a thoracic surgery centre and histology revealed parathyroid adenoma embedded within thymic tissue. Calcium and parathyroid hormone normalized after surgery. This case highlights the challenges in diagnosis, investigation and management of hypercalcemia due to PHPT during pregnancy. The diagnosis is often delayed and needs a high index of suspicion. In our case, detailed probing revealed an 18-month history of generalized weakness, polyuria, nocturia, vomiting, and memory impairment. Localization of abnormal parathyroid gland/s is difficult in pregnancy as ultrasound is the only safe imaging technique and, when used alone, has low sensitivity. Most calcium-lowering drugs are contraindicated, with rehydration being the mainstay of treatment. If parathyroidectomy is indicated, it is best performed in the second trimester when organogenesis is complete and the risk of preterm delivery relatively low.