Low free thyroxine in infants may be associated with fatigue, lethargy, pallor, constipation, dry skin, and in severe cases, anemia and heart failure. Low free thyroxine in infants may be associated with hypothyroidism: 1. If the test reveals that the free thyroxine is low, but the thyroid stimulating hormone level is normal and there is no other adverse reaction, hypothyroidism can be ruled out. 2. On the contrary, if the examination finds that the infant’s free thyroxine is low, but the thyrotropin level is elevated, accompanied by symptoms such as drowsiness, pallor, constipation, dry skin, anemia, heart failure, etc., it may be hypothyroidism. It is recommended that infants with these symptoms seek medical attention to identify the cause of the disease and treat it in a timely manner.