PTH and Calcium

We received a call from Endocrinology at CHOP today. Lily's PTH levels are high, and her Calcium levels are low. They want to start her on 500mg of calcium carbonate daily and retest in 2 weeks. If her Calcium levels don't come up, then they may have to order more testing to find out why? Have any other HLHS families had experience with low calcium or high PTH?