Background: Thyrotropinomas (TSHomas) have traditionally been considered a rare, albeit important cause of thyrotoxicosis. However, a recent report suggests their prevalence may be 23 times higher than previously suspected. In addition, although early case series described a predominance of invasive macroadenomas, recent findings (including in our own cohort of 40 patients) confirm that microadenomas are being increasingly diagnosed, and the clinical/biochemical phenotype appears to be more variable than previously reported.
Investigation: When faced with a patient with raised thyroid hormones (TH) and apparent inappropriate (non-suppressed) TSH, the first step is to exclude potential confounding medications (e.g. heparin, amiodarone) or intercurrent illness (including acute psychiatric disorders). Thereafter, it is important to screen for assay interference (e.g. due to heterophilic antibodies or familial dysalbuminaemic hyperthyroxinaemia (FDH)) to avoid unnecessary further investigation. Once a diagnosis of genuine hyperthyroxinaemia with inappropriate TSH secretion has been confirmed, a complex pathway of investigation is often required to reliably differentiate between TSHoma, resistance to thyroid hormone due to loss-of-function mutations in the beta isoform of the TH receptor (THRB RTH), and rare disorders of TH transport or metabolism. Undue reliance on pituitary MRI findings may result in inappropriate surgery for a pituitary incidentaloma in RTH, or failure to recognise a microTSHoma that is not readily visualised on standard MRI (equivalent to the microcorticotroph adenomas found in patients with Cushings disease).
Management: Transsphenoidal surgery is the mainstay of treatment for TSHoma. However, somatostatin analogue (SSA) therapy is finding increasing use, both in the preparation of patients for surgery, but also as an adjunct following incomplete tumour resection, or even as primary medical therapy. Radiotherapy remains an important option in more aggressive/invasive tumours.