Effectiveness of thrice-daily short-acting somatostatin analog testing in patients with thyrotropin-secreting pituitary neuroendocrine microtumor: A case report with review of literature

The laboratory diagnosis of thyrotropin-secreting pituitary neuroendocrine tumors (TSH-PitNETs) is characterized by the syndrome of inappropriate secretion of thyrotropin (SITSH), which presents as non-suppressed TSH levels despite hyperthyroxinemia. Differential diagnoses include resistance to thyroid hormone β (RTHβ) and a syndrome that is clinically and biochemically indistinguishable from RTHβ but without THRB mutations (non-TR-RTH). A 53-year-old Japanese woman without a family history of thyroid disease was referred to our hospital with suspected SITSH. THRB gene analysis revealed no mutations. Magnetic resonance imaging described a pituitary nodule with a maximum diameter of 8.5 mm, raising the suspicion of TSH-PitNET, or a combination of non-TR-RTH and pituitary incidentaloma. Among multiple evaluations we conducted, a 100 μg thrice-daily short-acting somatostatin analog octreotide suppression test significantly suppressed her TSH levels, indicative of a TSH-PitNET, which was confirmed postoperatively. However, the inconvenience of midnight octreotide injections in this protocol prompted further investigation into once- or twice-daily dosing regimens. A retrospective study was conducted on the suppressive effect of a single 100 μg dose of octreotide on TSH in 28 patients with acromegaly and three patients with TSH-PitNET who visited our hospital between 2008 and 2023. Significant differences in the suppressive effects at 12 h and 24 h post-injection suggest that a less frequent dosing regimen could be effective in diagnosing TSH-PitNET. In conclusion, this case report presents the first documented use of a thrice-daily octreotide test for diagnosing TSH-PitNET in Japan. We also discuss a potential less frequent octreotide dosing schedule based on our preliminary findings.