Total T4 (thyroxine)
Sum of bound and free thyroxine, the main thyroid hormone in the blood
Also known as
Definition
Thyroxine (T4) is the main hormone the thyroid makes; to take effect, it is converted to T3 in the body. Almost all T4 in the blood is bound to transport proteins and serves as a reserve, while only a small share is free. Total T4 measures both parts. Because the amount of transport proteins changes, for example during pregnancy or with birth control pills, free T4 is usually preferred today.
| Parameter | Value |
|---|---|
| Unit | µg/dl (nmol/l) |
| Reference Range | The sources used here do not give a single range; laboratories use their own ranges depending on the method. The reference range on your own report is what counts, and your doctor interprets it. |
Agree on your personal target range with your doctor.
Conversion: 1 µg/dl equals about 12.9 nmol/l.
↓ What a low value means
A low value can be linked to an underactive thyroid, for example with Hashimoto's disease, certain stages of thyroiditis, too little or too much iodine in the diet, rarely a pituitary problem or congenital hypothyroidism. Treatment for an overactive thyroid or thyroid cancer can also lower T4. Because total T4 includes the bound reserve, changes in transport proteins can shift the value without any change in thyroid function.
↑ What a high value means
A high value can be linked to an overactive thyroid, for example with Graves' disease, toxic goiter or a toxic thyroid nodule, certain stages of thyroiditis, a lot of iodine from medicines or seaweed, or taking too much thyroid hormone medicine. Rarely, a noncancerous pituitary tumor is the cause. More transport proteins, for example during pregnancy or with birth control pills, also raise total T4. Discuss abnormal results with your doctor.
When to test
T4 is usually measured together with TSH: with an abnormal TSH, signs of an underactive or overactive thyroid, thyroid disease in the family, to monitor hormone treatment, and in newborn screening. Special preparation is usually not needed; if other tests are run on the same sample, fasting may be required. Mention your medicines, but do not stop anything without talking to your doctor.
Frequently asked questions
Total T4 or free T4: which is measured more often? +
Usually free T4. It is considered more accurate because total T4 also captures the bound hormone and changes with the amount of transport proteins. A free T4 index (FTI) can be calculated from total T4.
Why do birth control pills change total T4? +
Birth control pills and pregnancy often change the amount of transport proteins in the blood. This changes the measured bound T4. In this situation, free T4 reflects thyroid function more accurately.
Is a T4 value enough to assess the thyroid? +
No. According to MedlinePlus, T4 alone does not provide enough information and is usually combined with TSH. The pituitary gland makes more TSH when T4 is too low and shuts TSH down when T4 is too high.
What affects T4 besides the thyroid? +
Among other things, severe illness, birth control pills, steroids and larger amounts of iodine. Abnormal values therefore do not always mean a condition that needs treatment; further tests are often needed.
Total T4 (thyroxine)
Sources
Last updated: October 05, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.