Nocturnal calcium, phosphorus and parathyroid hormone in the diagnosis of concealed and subclinical hypoparathyroidism.

CONTEXT Circadian rhythms of plasma parathyroid hormone (PTH) show peak values at night, whereas serum calcium levels peak in the evening and display a nadir at night. HYPOTHESES Subclinical hypoparathyroidism (HPT) can be detected by utilizing the knowledge of diurnal variations. Thalassemia major (TM) may provide a model system of subclinical HPT. DESIGN Nocturnal plasma PTH and serum calcium values were determined in 13 TM patients with normal morning serum calcium levels as compared with the corresponding values in eight healthy control subjects. RESULTS Six patients with TM presented a nadir serum calcium level of 8.3 mg/dl or lower (hypoCa TM) at 0200 h, whereas the remaining seven showed nadir levels of 8.4 mg/dl or higher (normoCa TM). Patients with hypoCa TM displayed a drop between peak and nadir of 1.2 +/- 0.5 mg/dl as compared with a considerably smaller fall of 0.3 +/- 0.7 mg/dl in control subjects (P < 0.05). NormoCa TM patients experienced comparable nocturnal variation to that of control subjects. Patients from both the hypoCa and normoCa TM groups presented significantly lower nocturnal PTH levels than those of control subjects and lost the nocturnal PTH variation characteristic of healthy subjects. A plot of all serum calcium against plasma PTH levels provides a clear distinction of the three groups. CONCLUSIONS All 13 daytime normocalcemic TM patients presented a certain degree of HPT. The hypoCa TM group displayed a concealed HPT detected in all, except the morning sampling, whereas normoCa TM patients experienced sub clinical HPT observed in the absence of nocturnal HPT variation. Nocturnal measurements of serum minerals thus enhance the sensitivity of HPT diagnosis.