Introduction: Thyrotropin-secreting adenomas are a rare cause of hyperthyroidism. Preferred treatment is pituitary neurosurgery.
Case report: A 35 year old lady was first noted to have abnormal thyroid function when presenting with an AV-nodal re-entry tachycardia in 2011. She had tremors, sweating and irritability. FT4 was 35 pmol/l (10-20 pmol/l) with a non-suppressed TSH level of 7.5 U/l (026 U/l). Previous meningococcal meninigitis age 13. No family history thyroid dysfunction. Further tests: negative screen for heterophilic antibodies, elevated SHBG at 198 nmol/l, alpha-1 subunit 3.85. The α1SU/TSH ratio was 0.46 supporting the diagnosis of a TSHoma. MRI confirmed a 3mm anterior pituitary enhancing lesion. A TRH stimulation test showed a blunted TSH response with a normal insulin stress test. TSH, T4 and T3 failed to suppress with 100mcg of levothyroxine over 10 days. TRH genetic analysis was negative. She was then referred to the tertiary centre for neurosurgical consideration. She received 3 months of Octreotide LAR with little change in TSH or thyroxine levels. Pre and post octreotride dynamic imaging did not show any pituitary adenoma, therefore surgery was considered inappropriate and she had radioactive iodine for symptom control. Six months post 131I MRI pituitary showed a 2 mm adenoma but no lesion was evident after 18 months. Post 1311 she became clinically hypothyroid; FT4 3.0 with TSH 32.1. Stabilising on thyroxine 100 μg she was symptomatically better with FT4 19.7 and TSH 13.7.
Conclusion: Radioactive iodine is not commonly used for TSHoma but was used in this case as there was failure of response to somatostatin analogues and the lack of identification of a definite pituitary adenoma on dynamic imaging.