Free thyroxine (free T4, fT4) is the second pillar of diagnosing thyroid disease after TSH. It shows the fraction of hormone that is not bound to blood proteins and can act on cells. Unlike TSH, free T4 has no pronounced daily rhythm, but it is very sensitive to the timing of the levothyroxine tablet, to certain medicines and to the specifics of the laboratory method. Our editorial team explains how to prepare so that the figure on the form is reliable.
What free T4 actually measures
The thyroid gland secretes mainly thyroxine. In the blood, over 99% of T4 is bound to carrier proteins: thyroxine-binding globulin (TBG), transthyretin and albumin. Only the small free fraction, hundredths of a percent, enters cells, where it is converted into the more active T3.
That is why doctors prefer free T4 over total T4: total T4 changes together with the concentration of carrier proteins, even when gland function is normal. The free fraction theoretically does not depend on this, although in practice immunoassays do not always perfectly «separate» the free hormone from the bound one.
Free T4 is ordered together with TSH to clarify the type of disorder. A high TSH and low free T4 indicate overt hypothyroidism; a high TSH and normal T4 indicate subclinical hypothyroidism. A low TSH with high T4 is characteristic of thyrotoxicosis. If both values are low, the doctor considers pituitary problems.
Since the half-life of T4 is about a week, its level in the blood is more stable than TSH and has no noticeable daily rhythm. But this stability applies to the body's own hormone; taking synthetic thyroxine creates a short-term rise that must be taken into account.
Levothyroxine on the day of the test
After swallowing a levothyroxine tablet, the T4 concentration in the blood rises and peaks approximately 2–4 hours later, then slowly returns to baseline. If blood is drawn during this window, free T4 may appear elevated, and the doctor will mistakenly decide the dose is too high.
Therefore the general rule for patients on replacement therapy: on the day of the test the morning dose is taken after the blood draw. There is no need to skip it entirely; it is enough to bring the tablet along and take it right after the procedure, observing the usual interval before food.
Treatment monitoring is done 6–8 weeks after the start of treatment or a dose change, as recommended by the ATA working group (Jonklaas et al., 2014). Earlier than that, T4 and TSH have not yet reached equilibrium, and decisions based on such results may be premature.
If a person takes the drug irregularly and, a few days before the test, «makes up for» missed doses, a paradoxical picture may arise: free T4 is normal or high, while TSH is still elevated. It is better to be honest with the doctor about missed doses, this will save unnecessary examinations.
Biotin, heparin and other interferences
Free T4 is usually measured by a competitive immunoassay. In systems that use the streptavidin–biotin link, an excess of biotin from supplements gives a falsely high result. Combined with a falsely low TSH, this can completely mimic hyperthyroidism, which is described in the work of Li and colleagues (2017) and in an FDA warning.
The second known artifact is heparin. Even a small amount of it activates lipoprotein lipase, which releases free fatty acids while still in the test tube. These displace T4 from proteins, and the measured free T4 rises artificially. That is why in people receiving heparin the test is interpreted with caution.
| Factor | Direction of fT4 error | Comment |
|---|---|---|
| Biotin in high doses | False increase | Depends on the analyzer |
| Heparin | False increase | In vitro artifact |
| Taking levothyroxine before the draw | Real temporary increase | Take the tablet after the test |
| Amiodarone | Increase | Alters T4 metabolism |
| Phenytoin, carbamazepine | Decrease in some methods | Displacement from proteins, accelerated metabolism |
| Heterophile antibodies | Any | Suspected when there is a discrepancy with the clinical picture |
Less often there are antibodies to thyroid hormones or heterophile antibodies that interact with the reagents. If the test results clearly do not match the symptoms, the doctor may ask to check the sample by another method or in another laboratory.
Regarding biotin, the practical advice is simple: tell the doctor about all vitamin complexes and, a few days before the test, stop taking high-dose supplements. Ordinary multivitamins with a small biotin content rarely create a problem, but it is worth checking the composition on the label.
Conditions that change carrier proteins
Pregnancy and taking estrogens (in particular combined oral contraceptives) increase the TBG concentration. Total T4 rises with it, while the free fraction actually changes little; however, some immunoassays work less accurately under these conditions. Therefore for pregnant women the ATA recommends taking the method and trimester-specific ranges into account.
Androgens and anabolic steroids, on the contrary, lower the TBG level. As a result, total T4 falls, and without measuring the free fraction one may mistakenly suspect hypothyroidism. It is important for the doctor to know about taking such drugs, even if the person considers it a «sports» matter.
Severe acute illnesses, fasting and high doses of glucocorticoids also change hormone binding and metabolism. In hospitalized patients free T4 may be either decreased or increased without gland disease, so a routine test is better postponed until recovery.
Finally, the result depends on the analytical platform: reference ranges for free T4 differ noticeably between laboratories. For monitoring over time it is advisable to choose one laboratory and compare results only with its ranges.
A practical checklist before the test
Although for free T4 the time of day is not as critical as for TSH, these tests are usually done together, so it is better to follow the stricter rules. A morning visit on an empty stomach makes results reproducible.
- Come in the morning, after 8–12 hours without food; drinking water is allowed.
- Take levothyroxine on the day of the test after the blood draw.
- A few days beforehand, stop high-dose biotin (after agreeing with the doctor).
- Do not train intensely the day before, and get a good night's sleep.
- Report your medicines: estrogens, androgens, amiodarone, anticonvulsants, heparin.
- Postpone a routine test if you currently have an acute infection.
Before the procedure it is helpful to sit for a few minutes to calm down. Short-term stress has little effect on free T4 itself, but for TSH and other values, which are often taken from the same tube, a calm state matters.
The result is always assessed together with TSH and the clinical picture. An isolated «borderline» free T4 without TSH changes most often has no clinical significance, but the doctor decides on that.
If you have already received an unclear result, do not rush to change the drug dose yourself. It is better to repeat the test following the rules described above and discuss both results with an endocrinologist.
Editorial conclusions
Free T4 is a stable indicator, but it is easy to distort: a levothyroxine tablet taken an hour before the test, biotin from a «beauty» supplement or heparin can change the figure so much that the diagnosis changes too.
The most important rules are to draw blood before taking the morning dose, to pause biotin intake, to tell the doctor about hormonal drugs and to compare results in a single laboratory.
Free T4 should not be interpreted separately from TSH: only the pair of these values gives the doctor a complete picture of the state of the thyroid gland.
We also recommend reviewing our materials on preparing for TSH and free T3 tests, as well as on how anabolic steroids affect laboratory values.
References
- Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacement. Thyroid. 2014;24(12):1670–1751.
- Garber JR, Cobin RH, Gharib H, et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Thyroid. 2012;22(12):1200–1235.
- Ross DS, Burch HB, Cooper DS, et al. 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid. 2016;26(10):1343–1421.
- Li D, Radulescu A, Shrestha RT, et al. Association of biotin ingestion with performance of hormone and nonhormone assays in healthy adults. JAMA. 2017;318(12):1150–1160.
- Alexander EK, Pearce EN, Brent GA, et al. 2017 Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid. 2017;27(3):315–389.
- Rifai N, Horvath AR, Wittwer CT (eds). Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. St. Louis: Elsevier; 2018.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.
