Low free triiodothyronine may present with systemic symptoms such as chills and malaise, pericardial effusion and heart failure when the heart is involved, and coma with mucous edema in severe cases. There is individual variability in the clinical manifestations of patients with low free triiodothyronine. Free triiodothyronine is a sensitive indicator for the diagnosis of hyperthyroidism. Free triiodothyronine is measured independently of changes in the concentration and binding properties of its binding proteins, and it is a commonly used indicator of thyroid function in clinical practice. Hypothyroidism is often characterized by low free triiodothyronine. As the disease progresses, patients may develop symptoms such as slower heart rate, loss of appetite, weakness of the limbs, dry skin, slow reaction time, low skin temperature, joint pain, and in female patients, reproductive function may be affected. For low free triiodothyronine, it is recommended to seek medical attention and standardize treatment under the guidance of a doctor.