Endocrine Abstracts

Background: TSH-secreting pituitary adenomas (TSHomas) are rare functional pituitary tumours, with an incidence of 1–2 per million. They present diagnostic challenges, particularly in patients with pre-existing hypothyroidism, where abnormal biochemistry may be misattributed to inadequate replacement.

Case Presentation: A 72-year-old man with longstanding hypothyroidism and prostate carcinoma was referred to endocrinology by his GP in 2021 with persistently elevated TSH despite escalating levothyroxine doses. Biochemistry showed raised TSH with inappropriately normal/high free thyroid hormones across multiple laboratories, excluding assay interference. Family studies were normal. A mildly elevated α-subunit and raised SHBG prompted pituitary MRI, which demonstrated an 8 mm left-sided microadenoma. Levothyroxine was discontinued, after which TSH remained elevated with free hormones at the upper reference range. At pituitary MDT in April 2022, advice was to remain off levothyroxine, perform a TRH stimulation test, and continue surveillance. The TRH test demonstrated a blunted TSH response, consistent with autonomous secretion. The patient remained well until 2024, when palpitations and insomnia developed alongside a mild rise in free T3. Repeat MRI showed no progression. A further MDT deferred surgery and recommended somatostatin analogue therapy. In June 2025, Lanreotide was commenced, resulting in biochemical improvement and symptom resolution.

Discussion: This case illustrates the complexity of diagnosing TSHomas in the context of hypothyroidism. Exclusion of assay interference, α-subunit measurement, TRH testing, and pituitary imaging were essential in establishing the diagnosis. MDT input guided management, ensuring careful consideration of biochemical findings, clinical course, and surgical risk. Importantly, the case underlines the need to consider central causes of abnormal thyroid function in patients with discordant biochemistry, even when a background of primary hypothyroidism exists.

Conclusion: TSHomas should be considered in patients with discordant thyroid function tests. MDT-guided somatostatin analogue therapy can provide effective disease control, allowing deferral or avoidance of surgery in selected patients.