Total T4

This lab test helps to evaluate thyroid function. It is also used to see how well your medication is working when treating thyroid problems.

Total T4 Overview

Reviewed: April 22, 2014

Thyroxine (T4) is the most abundant of the thyroid hormones. There are two types of thyroxine which are free thyroxine and thyroxine binding globulin. Total T4 test measures both types. Total T4 is usually measured to aid in diagnosing hypothyroidism and hyperthyroidism. Total T4 is also used to monitor how well medications are working in treating hypothyroidism and hyperthyroidism.

Total T4 is measured in micrograms per deciliter (µg/dL). The normal range for total T4 is 4.5 - 12.5 µg/dL.


Blood draw


Fasting is not required before the test.

Your doctor may withhold your thyroid medication before the test.

What the results mean

Values that are above the normal value can be due to hyperthyroidism, pregnancy, and certain medications such as amiodarone, estrogens, and, methadone.

Values that are below the normal levels can be due to primary hypothyroidism, radioactive iodine (RAI), or antithyroid medication (propylthiouracil, methimazole), malnutrition, nephrotic syndrome, GI protein loss, and chronic liver disease.  Values that are below the normal levels can also be caused by certain medications such as phenytoin, carbamazepine, anabolic steroids, interferon, Interleukin-2, lithium.