SAT-471 Abnormal Thyroid Function Tests in Acromegaly: A Clinical Conundrum

Background: Abnormal thyroid function tests can be difficult to interpret in acromegaly. The differential includes growth hormone/IGF-1 effects on the thyroid gland, pituitary adenoma cosecretion of TSH and GH and acromegaly in the setting of resistance to thyroid hormone. Case: A 28 year old male initially presented to the Endocrine clinic following trauma workup for a fall. He was incidentally noted to have a pituitary macroadenoma (2.2 x 2.4 x 1.8 cm) with optic chiasm compression. The patient reported a history of headaches, blurry vision and joint pain with multiple physician visits for joint pain predating the trauma by 5-8 years. He mentioned changes in facial features and exam was notable for macrognathia, frontal bossing and a broad nasal bridge. He also mentioned enlarging hands and a delayed growth spurt in his early 20s. The diagnosis of acromegaly was confirmed by elevated IGF-1 (658 ng/mL, normal range 98-282) and oral glucose tolerance testing (GH 2.2 ng/mL initially and 1.2 ng/mL at 90 minutes, normal < 1). He was also noted to have elevated fT4 (2.4-2.9 ng/dL, normal range 0.70 - 1.70) and with normal TSH levels (0.77-0.98 mIU/L, normal, 0.27-4.20). He endorsed palpitations and diaphoresis but denied tremors or weight loss. Exam revealed tachycardia and diaphoresis but normal thyroid size and texture. Review of prior neck imaging (CT and MRI at time of initial trauma evaluation) described a normal appearing thyroid. Methimazole 30 mg daily and propranolol 10 mg TID were prescribed. A week later fT4 increased to 3.6 ng/dL and TSH 0.64 mIU/L. Free T4 by equilibrium dialysis (2.9-3.6 ng/dL) and T3 (207-242 ng/dL, normal range 71-180) were also elevated. Alpha subunit was 1.3 ng/mL (normal, < 0.55) and molar ratio of serum alpha subunit/TSH of 15 (normal < 0.3). TSI was < 0.10 units. Labs were suggestive of a TSH and GH co-secreting adenoma. To optimize thyroid function prior to surgery octreotide 50 mcg bid was prescribed. He underwent uncomplicated TSS resection and on POD1 revealed fT4 of 3.1 ng/dL. Immunohistochemical staining was positive for hGH and SSTR2a but negative for TSH. Conclusion: Once a diagnosis of acromegaly is made, it is important to evaluate the rest of the pituitary axis. Abnormalities in TFTs are common in this patient population and can be due to variety of etiologies including TMNG, co-secreting GH/TSH adenoma, TRH resistance and Grave’s disease. Interpreting TFTs can be challenging in these patients but understanding the physiology of GH, IGF-1 and thyroid hormones aids in diagnosis and treatment (1). Reference: (1) Gasperi, M.,Martino,E.,Manetti, L.,Arosio,M., Porretti,S.,Faglia, G., Mariotti, S. et al. 2002. “Prevalence of Thyroid Diseases in Patients with Acromegaly: Results of an Italian Multi-Center Study.” Journal of Endocrinological Investigation 25 (3): 240-45.