Concurrent Panhypopituitarism and Hyperprolactinemia Due to a Giant Internal Carotid Aneurysm Revealed by Thyroid Hormone Withdrawal During Follow-Up Management of Thyroid Cancer

ABSTRACT Objective: This article reports a case of secondary hypothyroidism occurring during the withdrawal of thyroid hormone replacement in a patient with thyroid cancer that led to the discovery of simultaneous panhypopituitarism and hyperprolactinemia that were caused by an internal carotid artery aneurysm. The discovery of the secondary hypothyroidism necessitated further pituitary hormone analysis and medical imaging to identify the precise pathologic etiology. Methods: The methods used included the evaluation of selected thyroid and pituitary hormone concentrations and subsequent pituitary magnetic resonance imaging. Results: An 86-year-old African-American female with a history of thyroid cancer exhibited a suppressed serum thyroid-stimulating hormone concentration that failed to increase during progressive lowering of her thyroxine dosage despite it causing a simultaneous subnormal free thyroxine concentration. Laboratory findings indicated secondary hypothyroidism and subsequently panhypopituitarism and hyperprolactinemia. Pituitary magnetic resonance imaging revealed the presence of a large carotid aneurysm that had invaded the sella turcica. Additional hormone analyses revealed elevated prolactin, low gonadotropins, low baseline cortisol within an inappropriately normal adrenocorticotropic hormone concentration, and a low cortisol concentration following cortrosyn stimulation. Conclusion: Panhypopituitarism and hyperprolactinemia were revealed in a patient with thyroid cancer during thyroid hormone withdrawal, leading to the discovery of a giant internal carotid artery aneurysm invading the pituitary fossa. Physicians should be alert to the possibility of secondary hypopituitarism when an expected rise in thyroid-stimulating hormone while tapering thyroid hormone replacement in a low risk thyroid cancer patient fails to occur. Abbreviations: FT4 = free thyroxine TSH = thyroid stimulating hormone MRI = magnetic resonance imaging GICAA = giant internal carotid artery aneurysm