- Role
- Main hormone released by the thyroid
- Paired with
- TSH
- Most common cause of low T4
- Hashimoto's disease
- Interference
- Biotin; estrogen affects total T4
What T4 does
T4 is something of a reservoir hormone. It's relatively weak on its own, but tissues around the body convert it into T3, the more active form, as they need it. Together they regulate metabolism, heart rate, temperature, digestion and energy.
Your pituitary gland controls how much T4 the thyroid makes by releasing TSH. When T4 drops, TSH rises to push the thyroid harder; when T4 is high, TSH falls.
Why labs prefer free T4
Most T4 in the blood rides on carrier proteins. Anything that changes those proteins changes the total T4 number, even if your thyroid is working normally. Estrogen from birth control pills, menopausal hormone therapy or pregnancy is the classic example: it raises carrier proteins and total T4. Free T4 sidesteps much of that problem, which is why it's the version most clinicians order.
How free T4 is read with TSH
Free T4 is almost always interpreted alongside TSH. The common patterns are:
- High TSH and low free T4: primary hypothyroidism, an underactive thyroid gland.
- High TSH and normal free T4: subclinical hypothyroidism, a milder, earlier stage.
- Low TSH and high free T4: hyperthyroidism, an overactive thyroid.
- Low TSH and low free T4: a less common pattern that can point to a pituitary problem.
- Low TSH with normal free T4 but high T3: possible T3-predominant hyperthyroidism.
Common causes of abnormal results
Low free T4 is most often due to Hashimoto's disease, an autoimmune condition that's especially common in women and becomes more frequent with age. High free T4 can come from Graves' disease, thyroid nodules, a phase of thyroiditis or simply too much thyroid medicine.
Severe illness, some medicines (including steroids) and very high or low iodine intake can also shift results. Biotin supplements can interfere with the test; stopping biotin for a couple of days before your blood draw avoids that.
If you take thyroid medicine
Levothyroxine, the standard treatment for an underactive thyroid, is a synthetic form of T4. Blood tests are usually repeated 6 to 8 weeks after starting or changing the dose, then every 6 to 12 months once stable. Take it consistently, typically in the morning before eating, and separate it from iron and calcium supplements, which can reduce absorption.
If you start or stop oral estrogen, let your clinician know. Estrogen changes thyroid-binding proteins, which may prompt a recheck of your levels.
Free T4: common questions
Is free T4 better than total T4?
For most people, yes. Free T4 is less affected by changes in blood proteins from estrogen or pregnancy.
Do I need to fast for a free T4 test?
Usually not, unless other tests drawn at the same time need fasting.
What does low free T4 with high TSH mean?
It's the typical pattern of an underactive thyroid, most often from Hashimoto's disease.
Can hormone therapy affect my thyroid tests?
Oral estrogen raises thyroid-binding proteins, which can raise total T4 and sometimes change your levothyroxine needs. Free T4 and TSH remain the best guides.
Should I take levothyroxine with my iron supplement?
No. Iron and calcium can block absorption, so take them several hours apart from levothyroxine.
How often should free T4 be checked on treatment?
Usually 6 to 8 weeks after a dose change, then every 6 to 12 months once your dose is stable.
Sources
We cite government agencies, professional societies and peer-reviewed research. See our editorial policy.
- Thyroxine (T4) TestMedlinePlus (U.S. National Library of Medicine)
- Thyroid Function TestsAmerican Thyroid Association
- Hypothyroidism (Underactive Thyroid)National Institute of Diabetes and Digestive and Kidney Diseases
- Iron: Fact Sheet for ConsumersNIH Office of Dietary Supplements