Endocrine Abstracts

Introduction: Silent somatotroph tumours are GH immunoreactive (IR) pituitary tumours without clinical and biological signs of acromegaly. In our series, they represent 16% of all the somatotroph pituitary tumours. The aim of our study was to compare the somatotroph tumours with and without acromegaly to a better characterization of these silent tumours.

Materials and methods: Fifty-nine tumours with acromegaly and 21 silent somatotroph tumours were studied. They were classified into monohormonal (pure GH) and plurihormonal (GH/PRL/±TSH) and into densely (DG) and sparsely granulated (SG) types. The proliferation (Ki-67 index, mitosis count), the differentiation (expression of somatostatin receptors SSTR2A-SSTR5 and Pit-1) and the secretory activity (% of GH IR cells) were compared in the 2 groups of patients.

Results: The silent somatotroph tumours were more frequent in women than in men (P<0.01), most of them (76% versus 35%) were plurihormonal (P<0.01), more frequently SG (67% versus 46%) and showed a lower percentage of GH IR cells (P<0.0001) than the tumours with acromegaly. In somatotroph tumours with and without acromegaly, the expression of both SSTR2A-5 was significantly lower in SG tumours than in DG ones (P<0.0004 and P<0.02 respectively). The silent plurihormonal tumours were larger (P<0.01), had a lower percentage of GH IR cells (P<0.0001), a higher Ki-67 index (P<0.006) and a lower expression of Pit-1 (P<0.002) than the plurihormonal tumours with acromegaly.

Conclusions: The silent somatotroph tumours are different from the tumours with acromegaly. However, the monohormonal GH tumours with and without acromegaly are very similar. In contrast, the frequent silent plurihormonal tumours are less differentiated and more proliferative than the plurihormonal tumours with acromegaly. A low secretory activity of these tumours might explain the normal plasma values of GH and IGF1 and the absence of clinical signs of acromegaly.