Endocrine Abstracts

Background: A recent study by the author suggests up to 73% of growth hormone (GH) secreting pituitary adenomas produce multiple hormones on histological analysis and 16% immunostain positive for TSH. We report the case of an acromegalic patient with a plurihormonal adenoma where diagnosis of thyrotoxicosis was made several years post-hypophysectomy

Case history: Mr JT was diagnosed with acromegaly at the age of 59 having been noted to have acromegalic physical features and failure to supress GH on a GTT. Baseline pituitary function showed elevated IGF-1 (493 mg/l) and prolactin (1943 mIU/l), normal TSH (0.96 mU/l), LH (3.5 IU/l) and FSH (2.9 IU/l) slightly elevated total thyroxine (170 nmol/l, normal range 70–140) and borderline low testosterone (10.1 nmol/l) pituitary MRI demonstrated a 4 cm macroadenoma and he underwent transphenoidal hypophysectomy. The adenoma was positive for GH, prolactin, TSH, LH and FSH on immunostaining.

Several months post-operatively he was noted to be in atrial fibrillation. He was chemically treated and underwent successful DC cardioversion but had recurrent episodes necessitating further cardioversions over an 18 month period. Clinical and biochemical thyrotoxicosis was evident with a free-T₄ of 24.3 nmol/l and free-T₃ of 7.2 nmol/l and carbimazole was commenced. TSH was not suppressed (1.26 mu/l). GH levels remained elevated and further therapy was initiated for his acromegaly (bromocriptine, followed by lanreotide). Eight years on, GH and IGF-1 levels are satisfactory. He remains on a small dose of carbimazole and is euthyroid.

Conclusion: A small proportion of GH adenomas co-secrete TSH. It is useful to identify these patients as they may develop clinical thyrotoxicosis although this is rare. Free-T₄ levels at the upper end of the normal range with a normal TSH should be viewed with suspicion. Where there is residual functioning tumour despite surgical and medical therapy, carbimazole in addition to somatostatin analogues may be used to maintain euthyroidism.