Endocrine Abstracts

Background: Thyrotropin-secreting pituitary adenomas (TSH-omas) account for 1% of pituitary tumours. In addition to TSH, these tumours frequently secrete glycoprotein α-subunit (αSU) and up to 50% are immunoreactive for growth hormone (GH), although GH suppression following oral glucose tolerance testing (OGTT) has not been routinely performed in all patients to identify subclinical GH hypersecretion or abnormal GH suppression.

Objectives: To examine GH co-secretion in TSH-omas using IGF-1 and OGTT.

Subjects and Methods: We studied GH dynamics of 5 patients with TSH-oma referred to our institution (2000–2005). The diagnosis of TSH-oma was made on the basis of elevated free thyroid hormones and an inappropriately normal TSH. TRH testing, αSU and αSU/TSH ratio were used to confirm the diagnosis. IGF-1 was measured in all patients. OGTT (75 g glucose) with GH measurement was performed in 4/5 patients. Subclinical acromegaly was defined as a nadir GH >0.5 mU/L following an OGTT and an IGF-1 above the normal range for age and sex. Abnormal GH suppression was defined as a nadir GH >0.5 mu/L and a normal IGF-1. Immunohistochemistry was performed in all patients following pituitary surgery.

Results: The mean fT4 was 46.8, range 31.2–57.8 pmol/L (NR 9.0–25.0); TSH mean 3.46, range 2.66–5.14 IU/L (NR 0.30–5.50). All patients had a macroadenoma and were immunopositive for β-TSH. Nadir GH following an OGTT was >0.5 mU/L in 4/4 patients. IGF-1 was elevated in 2/5 patients, one of whom had subtle clinical features of acromegaly. 3/5 patients were positive for GH on immunohistochemistry.

Conclusion: It is recognised that TSH-omas frequently demonstrate immunoreactivity for GH in the absence of clinical features of GH hyper-secretion. Our data demonstrate that immunoreactivity for GH may be accompanied by abnormal GH secretory dynamics.