Secondary Hyperthyroidism due to an Ectopic Thyrotropin-Secreting Neuroendocrine Pituitary Tumor: A Case Report

The main differential diagnoses of secondary hyperthyroidism include thyrotropin-secreting neuroendocrine pituitary tumors (TSH-PitNETs) and resistance to thyroid hormone. As a rare cause of secondary hyperthyroidism, ectopic thyrotropin-producing neuroendocrine pituitary tumors must also be considered.

A 48-year-old female patient with overt hyperthyroidism and elevated thyrotropin was admitted to the endocrine outpatient clinic of a secondary care hospital in March 2018. The patient had an inconspicuous pituitary MRI and F18-F-DOPA PET-CT, but showed a tumor mass located at the pharyngeal roof. Most biochemical tests and an increased tracer uptake of the pharyngeal mass in a Ga68-DOTANOC PET-CT argued for the presence of an ectopic TSH-PitNET. After treatment with octreotide over 5 days and a consecutive normalization of free thyroxine and free triiodothyronine, the tumor was endoscopically resected. Histologically, the mass consisted of small partially spindle, partially polygonal monomorphic to mildly pleomorphic cells with immunoreactivity for thyrotropin and luteinizing hormone. Postoperatively, the patient required intermittent levothyroxine therapy.

Ectopic TSH-PitNETs represent an extremely rare cause for secondary hyperthyroidism. While the diagnostic process may be complicated by negative imaging studies of the pituitary gland, family history, biochemical tests, and functional imaging using gallium-labelled somatostatin analogues may be helpful in establishing the diagnosis.