SUN-330 - Racing Hearts and the Thyroid: Identifying True Hyperthyroidism - A Case Report

Introduction:

Tachycardia is a common clinical finding with broad possible etiologies. Hyperthyroidism is one etiology. Thyroid hormones regulate cardiovascular function by increasing heart rate and myocardial contractility. Conversely, systemic illnesses can alter thyroid hormone metabolism and hypothalamic-pituitary function, resulting in transient abnormal thyroid tests (TFTs). We present two cases of patients with tachycardia and transient low TSH, to illustrate diagnostic considerations and guide appropriate management.

Case Presentation 1:
A 25-yr-old female with history of tachycardia and anemia referred to the endocrine clinic for abnormal TFTs (low TSH=0.35 mIU/L (0.55–4.78) and normal free T4 = 0.90 ng/dL (0.89–1.76). She reports 1 year history of palpitations/tachycardia and she was evaluated by a cardiologist with normal EKG, Echocardiogram, and Holter monitor. She now also has a recent cold/cough due to a viral infection. No personal or family history of thyroid disease. Repeat testing showed normalization of TSH to 0.82 mIU/L (0.55–4.78) with normal FT4 0.99 ng/dL (0.89–1.76) and FT3 3.60 pg/mL (2.30–4.20). Thyroid peroxidase and thyroglobulin antibodies were negative. Thyroid ultrasound revealed a normal-sized gland with homogeneous echotexture, normal vascularity, and no nodules.

Case Presentation 2:
38-yr-old female presented to the Emergency Department with chest pain and palpitations (HR 105) following physical exertion. Although physical exam was normal, labs revealed TSH of < 0.010 mIU/L (0.55–4.78), FT4 2.38 ng/dL (0.89–1.76), FT3 7.803.60 pg/mL (2.30–4.20). Endocrine was consulted. There was a strong family history of hyperthyroidism. Further labs showed TSI 114 IU/mL (normal < 0.55 IU/mL), Thyroid peroxidase 1087 IU/mL (normal < 34 IU/mL), and thyroglobulin Ab 52 IU/mL (normal < 20 IU/mL). Graves’ Disease was confirmed. She started with Methimazole and Atenolol therapy. Follow up labs revealed TSH of < 0.010 mIU/L (0.55–4.78), FT4 0.97 ng/dL (0.89–1.76), and FT3 3.42 pg/mL (2.30–4.20). There was notable clinical and biochemical improvement in 3 months.

Discussion:
Approximately 0.8% of patients who undergo thyroid testing for tachycardia receive a new diagnosis of hyperthyroidism or thyrotoxicosis. Tachycardia has a sensitivity of 70% and specificity of only 62% for hyperthyroidism, with a positive likelihood ratio of 1.8. [1] This means that tachycardia alone is a poor screening tool for hyperthyroidism, as many patients with tachycardia have other causes, and many hyperthyroid patients lack tachycardia.

References:
1. The utility of laboratory testing for suspected hyperthyroidism in the emergency department. PMID: 35131684.

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