Endocrine Abstracts

Case: A 44-year-old lady was diagnosed with Acromegaly (IGF-1 of 1315 ng/ml, prolactin 498 mu/l). She was also thyrotoxic clinically and biochemically with T₄ 29 pmol/l (normal 9-26), total T₃ 3.24 nmol/l (normal 0.9–2.2) but high TSH of 3.50 mu/l (normal 0.35–3.5). Further work-up with TRH testing and α-subunit measurement confirmed a TSHoma. Pituitary MRI scan showed a macroadenoma displacing optic chiasm and extending into cavernous sinuses bilaterally.

Following trans-sphenoidal surgery, the tumour remained clinically active (residual tumour 1.8 cm, IGF-1 and thyroid hormones raised with normal TSH). As gonadotropin and cortisol axes were normal she was started on octreotide LAR, which normalized her thyroid function. Acromegaly remained poorly controlled even when cabergoline was added (minimal IGF-1 582 ng/ml). We then changed her to the somatostatin analogue SOM230. Under an adapted dose of 900/600 μg/day, both thyroid function and the GH axis normalized after 12 weeks (IGF-I 179 ng/ml). There were no noticeable adverse effects but her symptoms and quality of life improved dramatically.

Discussion: Only 12 cases of a combined GH/TSHoma has been described before. Because of the preserved gonadotropic and corticotropic axis, we decided against radiation therapy. As somatostatin receptor subtypes (SSTR) two and five have been demonstrated in both Acromegaly and TSHomas, we treated the patient with Octreotide as a SSTR2/5 agonist. The partial response of GH secretion to Octreotide, suggests that SSTR subtypes may be distributed differentially on TSH and GH secreting cells or in case of GH/TSH co-secretion. TSH release is more sensitive to Octreotide. The successful switching of the patient to SOM230 suggests that at least GH producing cells express, either increased SSTR5 or SSTR1 in addition. In summary, SOM230 known to be more potent on GH expressing cells showed a high potency on TSH secretion in this rare combined GH/TSHoma.