Endocrine Abstracts

Dopamine agonists (DA) are first line therapy for Prolactinoma which normalises prolactin(PRL) level in 80% of cases at a median weekly dose of 1 mg. An accepted criterion of pharmacological resistance to DA is failure to normalize PRL levels. We report a case of aggressive macroprolactinoma that required 7 mg of Cabergoline to reduce prolactin despite radiological evidence of tumour shrinkage. A 42 year old male presented with a bitemporal field defect. Imaging confirmed an invasive macroprolactinoma. Investigations showed elevated PRL level of 91,760 mU/L and hypogonadotropic hypogonadism (FSH-3.3 u/L, LH-2.5 u/L; testosterone- 6.2 nmol/L). The patient was started on 500 mcg of Cabergoline/week and the dose was titrated to 1 mg/week. After 6 months there was marked reduction in the size of the tumour which was accompanied by a fall in prolactin to 10,6050 mU/L. Thereafter, prolactin level remained static and the dose of Cabergoline was progressively titrated to a maximum of 7mg weekly. Repeat MRI scan showed complete shrinkage of macroprolactinoma. Prolactin remained persistently elevated at 1,826 mU/l. This dose was associated with adverse effects and the dose of cabergoline was reduced gradually to a maintenance dose of 500 mcg weekly. Prolactin remains slightly elevated at 2037 mU/l but is stable with no associated increase in tumour size. This case highlights marked secondary resistance to Cabergoline following an initial favourable response. Secondary resistance to DA occurs rarely but this case demonstrates that effective tumour shrinkage can be obtained with higher doses of cabergoline with careful monitoring of adverse effects. Once tumour shrinkage has been achieved the dose of cabergoline should be reduced to the lowest effect dose that maintains a stable prolactin level. Complete normalisation of prolactin may not be feasible or indicated in the majority of cases.