Endocrine Abstracts

Case history: We present the case of a 64 year old man with an incidental finding of pituitary macroadenoma (1.1×0.8×0.5 mm). He had a past medical history of obstructive sleep apnoea, asthma, osteoarthritis, hypertension, retinal detachment and cataract.

Investigations: Blood tests.

Results and treatment: Pituitary function tests showed markedly elevated prolactin 4272 mIU/L (86–324), elevated free tri-iodothyronine (fT3) 18.2 pmol/L (3.1–6.8) and free thyroxine (fT4) 42.5 pmol/L (12.0–22.0) with a non-suppressed thyroid stimulating hormone (TSH) 1.08 mIU/L (0.27–4.20). Macroprolactin was not present in significant amounts. Repeat free thyroid hormones showed similarly elevated levels and a normal TSH. The patient was clinically euthyroid with no evidence of goitre or thyroid orbitopathy. There was no past medical or family history of thyroid disease, intercurrent (non-thyroidal) illness and medication usage including thyroxine, heparin, amiodarone. As the clinical picture was consistent with prolactinoma rather than a TSHoma or thyroid hormone resistance, we suspected assay interference. This was confirmed when free thyroid hormones were measured at another laboratory using different assays. The initial thyroid function tests were analysed with a Roche cobas analyzer and repeat tests showed free T3 3.25 pmol/L, free T4 10.4 pmol/L and TSH 0.81 Miu/L with an Abbott Core Immunoassay Analyzer.

Conclusions and points for discussion: We present a case of incidental prolactinoma and discordant thyroid function tests. It is important to consider the clinical context. Although TSH-oma and thyroid hormone resistance may have fit the biochemical picture, absence of overt manifestations of thyrotoxicosis made assay interference more likely. Correctly identifying assay interference avoided unnecessary further investigations.