HYPERPARATHYROIDISM IN PREGNANCY. R. Khurana 1 ; 1 University of Alberta, Edmonton, AB LEARNING OBJECTIVES: 1) Recognize the changes in calcium metabolism that occur with pregnancy. 2) Manage hyperparathyroidism in pregnancy. CASE INFORMATION: A 38-year old pregnant woman, G4P1 at 21 weeks gestation was seen for assessment of hypertension. Her blood pressure had been normal at the first prenatal visit, but had started to rise and had been high for 4 weeks. She was asymptomatic. Past medical history was significant for two miscarriages and one full-term pregnancy, all in Nigeria. There was no prior history of hypertension. Her only medications were prenatal vitamins. Physical exam was normal other than a blood pressure of 168/94 and a grade II/VI ejection systolic flow murmur. Laboratory screening for preeclampsia was normal. Laboratory screening for secondary causes of hypertension revealed hypercalcemia at 2.79 mmol/L (11.16 mg/dl) with an albumin of 36 g/L (3.6 mg/dl). An elevated ionized calcium confirmed hypercalcemia. A 24 hour urine calcium was elevated at 9.0 mmol/24h (360 mg/24h). 25-OH Vitamin D levels were normal, but her Parathyroid hormone (PTH) level was elevated at 15.0 pmol/L (normal 1.1 ± 6.8 pmol/L). She was told to keep herself well hydrated with oral fluids and started on oral phosphate 500 mg bid. A 24 hour urine for metanephrines was ordered to rule out pheochromocytoma as part of a multiple endocrine neoplasia (MEN) syndrome in combination with the hyperparathyroidism. It was normal and she proceeded to exploratory surgery with removal of a right superior parathyroid adenoma. Post-operatively, her calcium levels normalized. DISCUSSION: Primary hyperparathyroidism is rare during pregnancy. It may be masked because total calcium levels decrease secondary to a decrease in albumin from the volume expansion that occurs with pregnancy. However, ionized calcium levels remain unchanged despite decreases in PTH levels. This is likely due to the effects of PTH related peptide (PTHrp) which is found to be increased during pregnancy. PTH-rp may also help regulate fetal calcium levels which are higher than maternal levels. Maternal hyperparathyroidism can cause significant morbidity for the fetus and neonate. There is an increased risk for stillbirth, intrauterine growth restriction, premature labor and neonatal hypocalcemic tetany. The diagnosis is often made on a retrospective basis postpartum after the development of neonatal tetany. Surgical management is usually preferred as it causes less morbidity than medical management. However, hyperparathyroidism can be associated with pheochromocytoma in Type 2 MEN and this should be ruled out prior to surgery.