Can we trust PTH measurements?

Background: Primary hyperparathyroidism is a rare endocrine disorder in children. Plasma parathormone (PTH) can be detected by different methods, that vary in sensibility, specificity and reproducibility. Case report: We report the case of a 3-year-old child with a history of recurrent lower limbs pain, absent during the night. Her physically examination was unremarkable: her height and her weight were 93,2 cm (M/+1 SD) and 13,3 Kg (+1 SD), respectively, and she had no clinical signs of rickets. We assessed her bone mineral status, finding a very high plasma PTH value: 1493 pg/ml (normal range: 10-60 pg/ml, test LIAISON®, N-TACT® PTH) with 25(OH)vitamin D level of 28,5 ng/ml and normal values of calcium, phosphate and ALP; urinary ions resulted within the normal range too. The bone ultrasound at her finger showed normal value of quantitative ultrasound parameters (AD-Sos and BTT z-score). The ultrasound of the neck revealed a solid nodule of 6 to 4 mm at the level of the left-lower pole of the thyroid, which was interpreted as an hypertrophic parathyroid. So we decided to perform whole body MIBI-scintigraphy, that was normal, and the analysis of the following genes: NHERF1, GNAS, PTHR1, PTH and MEN, all of them resulting negative, while RET gene analysis showed an heterozygous polymorphism in the exon 11, without any clinical relevance. We recommended the patient to take vitamin D supplementation, but we obtained only a partial and temporary decrease in PTH value (246 pg/ml). Anyway, the subsequent determination of plasma PTH, that was performed by another method (DiaSorin LIAISON 1-84), revealed a normal PTH value (17,6 pg/ml), despite the persistence of the parathyroid nodule. Conclusions: Since the sensitivity of plasma PTH determination is poor, we recommend to repeat the analysis with different methods, if the results are inconsistent with the other laboratory, instrumental and clinical findings.