Prolactinomas are the most common pituitary adenomas. The treatment is primarily medical with dopamine agonists. The improvements in prolactin levels are accompanied by reduction in the tumour size in the majority of cases. We report a patient with a macroprolactinoma whose tumour enlarged despite achieving normal prolactin levels.
A 55-year-old gentleman was referred to the ophthalmologist with worsening vision in the left eye. Clinical examination revealed left afferent pupillary defect and atrophy of the left optic nerve. Pituitary fossa MRI showed a pituitary macroadenoma of 4.1 cm cranio-caudal diameter (CCD) extending into both cavernous sinuses and displacing the optic chiasm superiorly. Biochemical evaluation showed evidence of hypogonadotropic hypogonadism [testosterone 0.2 nmol/l (9.927.8), FSH 4.2 iu/l (111), LH 1.4 iu/l (18)], hypoadrenalism (peak cortisol level 291 nmol/l during synacthen test, normal response > 550) and evidence of growth hormone deficiency (peak growth hormone < 0.1 during glucagon stimulation test, normal response > 20). Prolactin was grossly elevated (130000 mu/l, normal: 0360). Thyroid function test was normal. A diagnosis of pituitary macroprolactinoma with anterior hypopituitarism was made. He was started on Hydrocortisone replacement and Cabergoline (500 micro-grams twice-weekly). Prolactin levels normalised within 2-months (43 mu/l) and a repeat MRI showed improvement in the macroadenoma with a reduction in the CCD to 2.9 cm. A follow-up MRI after 5 months showed an increase in tumour size to a CCD of 3.6 cm despite that the patient was still on Cabergoline (500 micro-grams twice-weekly) and the prolactin levels were still normal (24 mu/l).
Normalisation of prolactin levels in patients with macroprolactinoma does not necessarily correlate with tumour shrinkage. Therefore, follow-up imaging is important irrespective of normalising prolactin levels. Despite normalisation of prolactin levels in patients with macroprolactinoma, tumour shrinkage is not always the rule, on the contrary tumour size may expand or remain unchanged.