Pituitary Macroadenoma Masked by Iatrogenic Adrenal Insufficiency: A Diagnostic Blind Spot

Background: Exogenous steroid use is the most common cause of central adrenal insufficiency. Depending on the duration and strength used, it may take months to years for the hypothalamic-pituitary-adrenal (HPA) axis to recover after the steroid is stopped. We report a case of iatrogenic hypoadrenalism with persistent suppression of the HPA axis for 13 years, discovered later to be due to a second pathology. Case: A 48 year old lady presented in 2005 with weight gain of 20 kg over 1 year and florid Cushingoid features. 9AM cortisol was undetectable (<12 nmol/L). History taking revealed use of oral dexamethasone at various dosages over the past 9 years for her knee pains. A diagnosis of iatrogenic adrenal insufficiency was made. She was started on hydrocortisone replacement, and was advised to stop the over-the-counter steroids. By 2011 her short Synacthen test (SST) showed much improved functioning of the HPA axis (cortisol 182 (0 min) -> 329 (30 min) -> 408 nmol/L (60 min) [N peak>500 nmol/L]), and she was back to her usual body weight. However, subsequent monitoring revealed declining trend of 9AM cortisol from 135 nmol/L (2013) -> 99 nmol/L (2014) -> 57 nmol/L (2015) -> 64 nmol/L (2016) -> 18 nmol/L (2017) [N 166–507 nmol/L]. Hydrocortisone compliance and abstinence from exogenous steroids was confirmed with the patient. The ongoing hypofunction of the HPA axis was continually attributed by multiple physicians to her history of prolonged use of dexamethasone. In 2018, at the age of 60, the lady presented with new onset headaches, blurred vision, and bitemporal hemianopia for 3 months. MRI showed a 1.8x1.8x3.5 cm (WxAPxH) pituitary mass with suprasellar extension compressing the optic chiasm. Blood tests revealed panhypopituitarism: SST cortisol 17 -> 59 -> 49 nmol/L, ACTH 2.7 pmol/L [N <10.1 pmol/L]; fT4 9.5 pmol/L [N 12–22 pmol/L], TSH 0.97 mIU/L [N 0.27–4.2 mIU/L]; LH <0.1 IU/L, FSH 0.52 IU/L (menopause at age of 48); IGF-1 27 µg/L [N 41–279 µg/L]; prolactin 17 mIU/L [N 102–496 mIU/L]. After partial excision of the mass her vision improved, but remained dependent on hydrocortisone and thyroxine supplements. The lesion was pathologically proven to be a pituitary macroadenoma. Discussion: This case presents the uncommon course of a patient who had almost recovered from iatrogenic hypoadrenalism, only to lapse back into worsened central adrenal insufficiency, as part of panhypopituitarism related to an undiagnosed pituitary mass. In retrospect, the unusually protracted state of hypocortisolemia and the atypical waxing and waning HPA axis should have alerted one to consider alternative etiologies at work. As LH-FSH and GH deficiencies commonly develop before ACTH and TSH deficiencies in most pituitary macroadenomas, a lower threshold for testing the other anterior pituitary hormones followed by imaging of the pituitary could have picked up the tumor earlier in this patient.