Immunohistochemical expression of somatostatin receptor subtypes 2 and 5 in thyrotropin-secreting pituitary adenomas: a consecutive case series of pituitary adenomas

Thyrotropin-secreting pituitary adenomas (TSHomas) are a rare cause of hyperthyroidism. Somatostatin analogue has proved to be effective for inhibiting pituitary hormones secretion, working via interactions with somatostatin receptors (SSTRs). We therefore determined the relative predominance of SSTR2 and SSTR5 subtypes among the different types of adenomas, especially TSHoma, investigated the relationship between efficacy of shortterm octreotide (OCT) treatment and SSTR expression. Patients were enrolled at Beijing Tiantan Hospital between 2009 and 2015. Serum hormone determinations and histological findings in resected tissue resulted in five diagnoses: 16 TSHomas, eight acromegaly, three prolactinomas, three corticotropinomas, and four clinically nonfunctioning adenomas (NFPAs), and four normal pituitary specimens. IHC was performed on formalin fixed and paraffin embedded tissue in tissue microarrays. IHC of SSTR subtypes in the different cohorts showed that SSTR2 staining intensity scores higher than SSTR5 in TSHoma, acromegaly and prolactinoma, whereas in corticotropinoma and NFPA, the expression of SSTR5 was stronger than SSTR2. SSTR2 and SSTR5 expression were significantly higher in TSHoma than in other pituitary adenomas. OCT treatment for a median of 8.4 days (range: 3-18 days) and with a total median dose of 1.9 mg (range: 0.9-4.2 mg) witnessed significant decrease of all patients’ thyroid hormone levels (TSH [μIU/ml]: 4.95 ± 3.59 to 0.92 ± 1.55 [t = 4.721, P = 0.000]; FT3 [Pmol/L]: 11.77 ± 8.69 to 4.17 ± 0.88 [t = 3.507, P = 0.003]; FT4 [Pmol/L] 29.56 ± 8.51 to 16.72 ± 4.13 [t = 6.662, P<0.01]) respectively. Patients with low SSTR5 expression presented a significantly higher TSH suppression rate (P values <0.05). The present data confirm that somatostatin analogs should be considered as a medical alternative to surgical treatment, especially in patients with TSHoma, and short-term response to OCT therapy may be related to the expression of SSTR5.