Thyroid Function Testing in Hospitalized Patients: A Practical Framework for Clinicians.

Thyroid-stimulating hormone (TSH) is the primary screening test for thyroid issues in outpatient settings; however, interpreting it in hospitalized patients is more complex. Acute illness, medication effects, and assay interference can significantly influence thyroid function tests, often without underlying thyroid disease. Non-thyroidal illness syndrome, also called euthyroid sick syndrome, is common in hospitalized and critically ill patients and frequently causes temporary changes in thyroid hormone levels that can mimic primary thyroid disorders. Understanding when to test thyroid function in hospitalized patients is crucial for proper interpretation and clinical decision-making. This narrative review examines the physiology of thyroid hormone action and how the hypothalamic-pituitary-thyroid axis changes in the hospital setting. We also summarize the clinical presentation and management of thyroid emergencies and highlight situations in which TSH testing is most useful. Additionally, we provide practical guidance on interpreting thyroid function tests in hospitalized patients and identify key clinical scenarios that require endocrinology consultation. Finally, we propose a practical diagnostic algorithm to help residents and medical students distinguish transient thyroid function abnormalities caused by acute illness and to guide appropriate evaluation and management in the inpatient setting.