THU097 Primary Overproduction Of TSH Presenting As Intermittent Atrial Fibrillation Responding To Methimazole

Disclosure: T.D. Maher: None. Background: Intermittent Atrial Fibrillation (IAF) is commonly associated with Hyperthyroidism. TRH from the hypothalamus stimulates TSH production in the pituitary; a negative feedback loop of thyroxine to the hypothalamus is the primary regulator of TSH production. The Pituitary is adjacent to the sphenoid sinus; nasal and sinus inflammation and other factors may affect TSH production. With currently available thyroid tests it is a clinical challenge to be certain the thyroid is not contributing to IAF. Clinical Case: A 72-year-old male presented at age 57 (2007) with new onset IAF. The patient had Allergic Rhinitis since age 20. For IAF he was treated with Metoprolol, Flecanide and Dofenilide but IAF continued. On 3/2/11, TSH was 5.12 ((0.270-4.2 uU/Ml). At age 64 (2014), Catheter Ablation was performed. Before the Ablation, 08/16/2013, the FT4 was 1.2 (0.90-1.70) ng/ml and TSH 3.99 (0.270-4.200) uU/ml. Two years post Ablation, on 1/19/2016, TFTs were: FT4 1.10 (0.90-1.70) ng/ml; TSH 5.42 (0.270-4.200) uU/ml and repeat TFTs on 3/15/16 were: FT4 1.24 (0.90-1.70) ng/dl; TSH 4.10 (0.270-4.2 uU/dl. The diagnosis of Borderline Hypothyroid was entertained. He was trialed on Thyroxine 25 mcg daily with gradual increase to 75 mcg. The patient complained of being light headed, weak and having diffculty concentrating and the TSH was still elevated. Thyroxine was discontinued. In 2017, IAF returned. In October 2019 the thyroid was 35-40 Gm., firm, and with small nodules on palpation and ultrasound. The TFTs on 10/01/2019 were: FT4 0.81 (0.75-1.54) ng/ml and TSH 5.15 (0.3-4.2) uIU/ml. To test whether overproduction of TSH was causing excess Thyroxine release and IAF, low dose Methimazole was started at 5 mg, four days a week. There have been no episodes of AF for the past three years while on Methimazole. TFTs on 10/19/2022 were: FT4 0.94 (0.75-1.54) ng/ml and TSH 7.11 (0.099-4.2) uiU/m). Conclusion: In the clinical setting of IAF, elevated TSH and normal FT4, overproduction of TSH by extrinsic stimuli should be considered. Thyroid partial blockade with Methimazole is safe, economical, and reversible and may give valuable information about whether the thyroid is causing episodic Atrial Fibrillation. Presentation: Thursday, June 15, 2023