Different responses to chronic somatostatin analogues in patients with central hyperthyroidism

Central hyperthyroidism is mainly due to two different causes, TSH-secreting pituitary adenoma (TSH-oma) and resistance to thyroid hormone in its pituitary variant, i.e. patients presenting with signs and symptoms of hyperthyroidism (PRTH). Because therapeutic approach and the clinical follow-up are extremely different in these two disorders, a correct differential diagnosis is mandatory. Unfortunately, no definite pathognomonic tool is presently available and an extensive biochemical and instrumental workup is frequently needed in order to reach the correct diagnosis. Aim of the present study was to investigate the use of somatostatin analogues in the differential diagnosis between TSH-omas and PRTH, as well as the possible treatment of PRTH with these analogues.

Eight patients with TSH-oma and four with PRTH underwent the acute test with somatostatin analogue Octreotide (0.1 mg subcutaneously), as well as long-acting Octreotide-LAR (30 mg intramuscularly every 28 days) for two months.

Serum TSH, FT3 and FT4 were evaluated in basal condition, at time 0 and every hour for 6 h during acute test, and every 15 days for 2 months during chronic treatment.

During acute test, in both patients with PRTH and TSH-oma, a similar reduction in immunoreactive TSH and FT3 levels was observed, while no variations were found in FT4 concentrations. In contrast, during the administration of Octreotide-LAR, no significant variations of all tested parameters were observed in PRTH group, whereas FT3 and FT4 concentrations normalized or presented a significant reduction (> 30% of pretreatment values) in seven of eight patients with TSH-oma, despite minor variation of immunoreactive TSH levels.

Chronic administration of long-acting somatostatin analogues in patients with central hyperthyroidism caused a marked decrease of FT3 and FT4 levels in all patients but one with TSH-oma, while patients with PRTH did not respond at all. Thus, administration of long acting somatostatin analogues for at least 2 months can be useful in the differential diagnosis in problematic cases of central hyperthyroidism. Furthermore, the present findings exclude the possibility of a beneficial effect of chronic administration of somatostatin analogues in controlling thyrotoxic symptoms in PRTH patients.