Thyroid hemiagenesis (TH) is characterized by the congenital absence of one thyroid lobe. The aim of this study was to evaluate the calcium-phosphate balance in TH. Twenty patients with TH and 20 controls with a bilobed thyroid were studied. Serum concentrations of total calcium, parathormon and calcitonin were measured. Additionally, the immunohistochemical expression of calcitonin, chromogranin A (chA), neuron-specific enolase (NSE) and calcitonin gene-related peptide (CGRP) was evaluated in surgical specimens from patients with TH and controls. There were no significant differences in biochemical parameters between TH and controls. Positive staining for calcitonin was demonstrated in 3/8 thyroid sections from three patients with TH, but only in 2/33 sections from four controls (p < 0.005). All sections from patients with TH positive for calcitonin also expressed chA, NSE and CGRP. Two sections from controls positive for calcitonin presented an additionally positive reaction for chA, and one of them also for NSE. None presented positive staining for CGRP. Of three TH sections, in one, hyperplasia of C cells of medium grade, and in another hyperplasia of C cells of high grade, could be detected. In the controls, hyperplasia of C cells of low and medium grade was observed. TH was associated with slightly enhanced C cells hyperplasia compared to controls, which might indicate compensatory proliferation. However, the calcium-phosphate balance does not seem to be significantly affected. (Folia Histochemica et Cytobiologica 2011; Vol. 49, No. 2, pp. 299&#8211;305
