La fracción activa no unida de la principal hormona tiroidea, siempre leída junto con TSH.
Otros nombres:Free thyroxine, FT4Sección:Tiroides
Actualizado: 29 de agosto de 2026·Equipo editorial de MedProtect Información de referencia — no es un diagnóstico ni una prescripción
En resumenA typical reference range is 0.8–1.9 ng/dL (10.3–24.5 pmol/L). The range is population-based, so symptoms and the TSH result still matter when the number falls inside it.
Valores de referencia
Grupo
Valor de referencia
Adultos
0.8–1.9 ng/dL (10.3–24.5 pmol/L)
Normal value ranges vary slightly among different laboratories, and some laboratories use different measurements or test different specimens. The range is population-based: an individual result is interpreted together with symptoms and the TSH level.
Compruebe su valor
Enter the number from your lab report — see how it compares with the typical range. Mind the units shown in the chosen row.
A rough comparison with a typical range, not a diagnosis. Reference ranges vary between laboratories — check the range on your own report and let a clinician interpret the result.
Qué es este marcador
T4 (thyroxine) is the main hormone produced by the thyroid gland. Most of it travels through the blood attached to carrier proteins, and the small unbound fraction — free T4 — is what this test measures. Because the free fraction is the part available to the body's tissues, it reflects thyroid function more directly than the total amount.
The test is recommended when other thyroid blood tests such as TSH or T3 come back abnormal, when there are symptoms of an overactive or underactive thyroid, when the pituitary gland may not be producing enough of its hormones (hypopituitarism), when a lump or enlargement of the thyroid is found, and in some cases of difficulty becoming pregnant. It is also used to monitor people being treated for thyroid problems.
Results are reported in nanograms per deciliter (ng/dL); in picomoles per liter (pmol/L) in many countries. A typical normal range is 0.8–1.9 ng/dL (10.3–24.5 pmol/L), but the result is never read alone: to fully understand a free T4 value, the results of other thyroid tests, such as TSH or T3, may be needed.
The normal range is based on a large population and is not necessarily normal for an individual — symptoms of hyperthyroidism or hypothyroidism can occur even while free T4 sits inside the range, and the TSH test may help determine whether symptoms are related to thyroid disease. Test results can also be affected by pregnancy, estrogen level, liver problems, more severe body-wide illnesses, and inherited changes in the protein that binds T4.
Cuando está elevado
Graves diseaseThe most common overactive-thyroid condition: the immune system stimulates the thyroid, and free T4 rises.
Too much thyroid hormone medicineAn excessive dose of thyroid hormone replacement pushes free T4 above the range.
ThyroiditisInflammation of the thyroid releases stored hormone into the blood.
Toxic goiter or toxic thyroid nodulesAutonomously working thyroid tissue produces hormone without waiting for the TSH signal.
Rare causesSome tumors of the testes or ovaries (rare), medical imaging with iodine-containing contrast dye (rare, and only if there is a thyroid problem), and eating a lot of iodine-rich foods (very rare, and only if there is a thyroid problem).
Cuando está bajo
HipotiroidismoAn underactive thyroid — including Hashimoto disease and other disorders involving reduced thyroid function — lowers free T4.
Severe acute illnessA serious body-wide illness can temporarily push free T4 down without primary thyroid disease.
Malnutrition or fastingInadequate nutrition reduces thyroid hormone production.
Ciertos medicamentosSome medicines lower free T4, which is why the provider reviews the medication list when interpreting the result.
Qué puede alterar el resultado
Estos factores pueden alterar el valor sin ninguna enfermedad — vale la pena considerarlos antes de repetir la prueba:
Biotin (vitamin B7) supplementsEmbarazoKidney and liver diseaseEstrógeno levelSevere body-wide illnessInherited changes in the protein that binds T4
Cómo prepararse
No special preparation is needed. In general, test results are not affected by medicines, but biotin (vitamin B7) can affect them — the provider needs to know if it is being taken. Pregnancy and kidney or liver disease can also affect the result.
Cuándo tiene sentido repetir la prueba
A free T4 result is interpreted together with TSH and, when needed, T3 — the combination separates thyroid causes from pituitary ones. During treatment for a thyroid problem, the test is repeated on the schedule set by the treating provider to track the response.
Quién suele revisar este resultado
Un resultado fuera del rango de referencia suele ser revisado por:an endocrinologist; your GP or primary care doctor for the first evaluation. Traiga su informe de laboratorio y cualquier resultado anterior a la cita.
Nombres usados en los informes de laboratorio
Su informe puede usar cualquiera de estos nombres: T4 libreT4Free thyroxineFT4Thyroxine test por equilibrium dialysis
Unidades y conversión
Medido en:ng/dL
ng/dL=pmol/L
ng/dL × 12.87 = pmol/L. Example: 1.0 ng/dL = 12.9 pmol/L. Results are rounded to 2 decimal places.
¿Varios marcadores en su informe?
See how MedProtect explains results in the app: the whole report, a check of the recognized values, and a clear breakdown.
In general they do not, but biotin (vitamin B7) can affect the results. Pregnancy, estrogen level, liver problems, severe illness, and inherited changes in the T4-binding protein can also shift the number.
Can free T4 be normal while symptoms persist?
Yes. The normal range is based on a large population and is not necessarily normal for an individual. Symptoms of hyperthyroidism or hypothyroidism can occur with an in-range free T4, and the TSH test may then help determine whether the thyroid is involved.
What is the difference between free T4 and total T4?
Total T4 counts all thyroxine in the blood, bound and unbound; free T4 counts only the unbound, active fraction. The free fraction is less affected by changes in binding proteins, which is why it is the preferred measurement.
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