What does a blood test for the five thyroid items check?

Blood tests for the five thyroid function items primarily measure T3, T4, free T3, free T4, and thyroid-stimulating hormone (TSH), serving as a fundamental assessment of thyroid function.
Through these five thyroid tests, elevated T3 levels are usually caused by multiple thyroid nodular enlargements, while decreased levels are typically associated with hypothyroidism, liver cirrhosis, nephrotic syndrome, and similar conditions. Elevated T4 levels are commonly due to hyperthyroidism, pregnancy, or oral contraceptive use; reduced levels mainly result from hypothyroidism or iodine-deficiency goiter.
If free T3 and free T4 levels are elevated, this is usually due to hyperthyroidism or thyroid hormone resistance syndrome; decreased levels are primarily seen in hypothyroidism or following the use of glucocorticoid medications. Patients with primary hypothyroidism typically show markedly increased TSH levels, whereas patients with hyperthyroidism exhibit significantly reduced thyroid hormone levels, sometimes even dropping below 0.