Introduction: Non-thyroidal illness syndrome commonly occurs during acute systemic illnesses and is characterized by suppressed or inappropriately normal thyroid-stimulating hormone with low or low-normal thyroid hormone levels. This biochemical pattern may obscure or delay the diagnosis of underlying hyperthyroidism, especially in mild or borderline cases. Acute stroke is a recognized trigger of non-thyroidal illness syndrome, yet its masking effect on hyperthyroidism is under-recognized in clinical practice. Case: A 58-year-old Malay male with cardiovascular risk factors presented with an acute ischemic stroke involving the left occipital lobe and right internal capsule. He developed new-onset atrial fibrillation (CHA₂DS₂-VASc 3) and mild left ventricular systolic dysfunction. Initial thyroid function tests during the acute stroke phase showed mildly suppressed thyroid-stimulating hormone (TSH) (0.30 mIU/L) with high-normal free T4 (21.2 pmol/L), interpreted in the context of acute illness. The patient was clinically stable without overt thyrotoxic features. One month post-stroke, repeat testing revealed further TSH suppression (0.09 mIU/L) and rising free T4 (27.6 pmol/L). Detailed history uncovered prior Graves’ disease in 2019 with treatment default. Examination revealed a small diffuse goiter without ophthalmopathy. The biochemical evolution following recovery from acute illness confirmed recurrent hyperthyroidism previously masked by nonthyroidal illness syndrome. Conclusion Acute stroke can induce cytokine-mediated suppression of the hypothalamic-pituitary-thyroid axis and altered peripheral deiodination, leading to misleading thyroid function tests. In this case, non-thyroidal illness syndrome blunted the biochemical severity of hyperthyroidism, delaying recognition despite high-risk features such as atrial fibrillation and prior Graves’ disease. Reliance on a single thyroid function test during acute illness may therefore result in underdiagnosis.
Euthyroid Sick Syndromes ; Ischemic Stroke ; Hyperthyroidism