A case of severe neonatal hypercalcaemia secondary to neonatal thyrotoxicosis

A 3-week-old male infant presented with poor feeding, lethargy, and weight loss. Clinical examination revealed mildly elevated tachycardia, but otherwise there were no clinical signs evident. Initial investigations revealed hypercalcaemia, which was subsequently found to be secondary to transient thyrotoxicosis. Hypercalcaemia was managed with hyperhydration along with a low-calcium infant formula, and thyrotoxicosis responded to treatment with carbimazole. Serum calcium levels gradually normalised with resolution of the neonatal thyrotoxicosis and weight gain. Subsequently, all treatment was gradually weaned over 4 months. The infant remained clinically well on follow-up with normal growth, and his laboratory investigations normalised.

Hypercalcaemia is a recognised complication of thyrotoxicosis. Evidence in the paediatric population is limited, and this is the first reported case of hypercalcaemia noted to occur secondary to neonatal thyrotoxicosis. Signs and symptoms of both hypercalcaemia and thyrotoxicosis can be non-specific and subtle. An awareness of the condition and a high index of suspicion are required in order to make the diagnosis. While asymptomatic neonatal hypercalcaemia may be transient and self-limiting, symptomatic patients require treatment with hyperhydration. Serum calcium levels in combination with parathyroid hormone, vitamin D, and urine calcium-creatinine ratios can help narrow the differential diagnoses, but it is important that the underlying cause is carefully investigated.