Background: Thyrotropin secreting adenomas (TSHoma) are considered to be rare pituitary tumours. However, improvements in imaging techniques and greater use of more sensitive thyrotropin (TSH) assays has led to a recent increase in the detection rate of TSHomas and, specifically, the identification of more microadenomas. Surgery is considered the mainstay of treatment, however, primary medical therapy with somatostatin analogues is an emerging alternative therapeutic option.
Methods: 10 patients with TSHomas were studied at baseline and 3 months after starting long-acting somatostatin analogues. Investigations included measurement of thyroid function tests, metabolic parameters and a panel of biomarkers of thyroid hormone action. Body composition was assessed using Dual-energy X-ray absorptiometry (DXA), and Whole Body Resting Energy Expenditure (REE) measurement was performed using a ventilated hood indirect calorimeter the morning after an overnight fast. Participants were fitted with an ‘Actiheart’ device to allow estimation of average sleeping heart rate. Pituitary imaging was performed using volume MRI and PET-CT.
Results: Free thyroid hormones normalized in 8 patients. TSH either normalized (n=6) or became frankly suppressed (n=4), with two patients requiring supplementation with levothyroxine as part of a ‘block and replace’ regimen. Significant changes in creatine kinase (P<0.05), sex hormone binding globulin (P<0.05) and total cholesterol (P<0.05) were observed, together with a significant reduction in REE (P<0.005) and sleeping heart rate (P<0.05). The majority of patients (n=7) had either osteopaenia or osteoporosis at baseline. Reduction in tumour size was only observed in 2 cases, but PET tracer uptake diminished in 8.
Conclusion: Somatostatin analogues are very effective in controlling hyperthyroidism in patients with TSHomas, leading to favourable changes in thyroid hormone dependent target tissues, such as heart, muscle, and liver. Unlike in patients with acromegaly, tumour size does not change significantly after 3 months of treatment in the majority of patients.