Thyroxine Test (T4)

What is Thyroxine Test?

The thyroid is a gland in the lower front of the neck. It makes hormones that affect many body parts and systems, including the system that affects how quickly the body burns fuel for energy (metabolism).

You may have a thyroxine test:

  • To help manage treatment for an underactive thyroid (hypothyroidism) or an overactive thyroid (hyperthyroidism).
  • To help diagnose hypothyroidism or hyperthyroidism. If you have possible symptoms of these conditions, you may have this test done.
    • Symptoms of hypothyroidism include:
      • Fatigue.
      • Weight gain.
      • Dry skin.
      • Cold intolerance.
    • Symptoms of hyperthyroidism include:
      • Tremors.
      • Weight loss.
      • Anxiety.
      • Heat intolerance.
  • If you are pregnant and have thyroid disease. You may have this test to make sure your hormone levels remain normal during pregnancy.
  • To help diagnose other conditions that affect thyroid function.

Newborn babies may have this test done to screen for hypothyroidism that is present at birth (congenital).

What is being tested?

This test measures the amount of total thyroxine (T4) in the blood. T4 is the main hormone made by the thyroid.

Some T4 is attached (bound) to proteins in the blood. Some remains free (free T4). Your health care provider may test you for total T4, free T4, or both.

What kind of sample is taken?

A blood sample is required for this test. It is usually collected by inserting a needle into a blood vessel. For newborns, a small amount of blood may be collected from the umbilical cord, or by using a small needle to prick the heel (heel stick).

How do I prepare for this test?

Follow instructions from your health care provider about changing or stopping your regular medicines. Many medicines can affect thyroid hormones, including birth control pills, estrogen, and aspirin.

Tell a health care provider about:

  • All medicines you are taking, including vitamins, herbs, eye drops, creams, and over-the-counter medicines.
  • Any blood disorders you have.
  • Any surgeries you have had.
  • Any medical conditions you have.
  • Whether you are pregnant or may be pregnant.
  • Any recent illness or stress.

How are the results reported?

Your test results will be reported as a value that indicates how much T4 is in your blood. Your health care provider will compare your results to normal ranges that were established after testing a large group of people (reference ranges). Reference ranges may vary among labs and hospitals.

Reference ranges for free T4 vary by age. Common reference ranges are:

  • 0–4 days old: 2–6 ng/dL or 26–68 pmol/L (SI units).
  • 2 weeks to 20 years old: 0.8–2 ng/dL or 10–26 pmol/L (SI units).
  • Adult: 0.8–2.8 ng/dL or 10–36 pmol/L (SI units).

Reference ranges for total T4 vary by age and gender. Common reference ranges are:

  • 1–3 days old: 11–22 mcg/dL.
  • 1–2 weeks old: 10–16 mcg/dL.
  • 1–12 months old: 8–16 mcg/dL.
  • 1–5 years old: 7–15 mcg/dL.
  • 5–10 years old: 6–13 mcg/dL.
  • 10–15 years old: 5–12 mcg/dL.
  • Adult male: 4–12 mcg/dL.
  • Adult female: 5–12 mcg/dL.
  • Any adult older than 60 years: 5–11 mcg/dL.
  • Pregnant female: 9–14 mcg/dL.

What do the results mean?

Results that are within your reference range are considered normal. This means that you have a normal amount of T4 in your blood.

Results that are higher than your reference range mean that you have too much T4 in your blood. This may mean that:

  • You have hyperthyroidism.
  • You have a thyroid condition called Graves’ disease.
  • You have other conditions that affect your thyroid function, such as thyroid cancer, thyroid goiter, or thyroiditis.
  • Your thyroid medicine dosage is too low.

Results that are lower than your reference range mean that you have too little T4 in your blood. This may mean that:

  • You have hypothyroidism.
  • You have problems with your pituitary gland function.
  • You have a thyroid condition called Hashimoto’s thyroiditis.
  • You have kidney failure.
  • Your thyroid medicine dosage is too high.

You will need more tests to confirm a diagnosis. Talk with your health care provider about what your results mean.

Questions to ask your health care provider

Ask your health care provider, or the department that is doing the test:

  • When will my results be ready?
  • How will I get my results?
  • What are my treatment options?
  • What other tests do I need?
  • What are my next steps?

Summary

  • The thyroid is a gland in the lower front of the neck. It makes hormones that affect many body parts and systems, including the system that affects how quickly your body burns fuel for energy (metabolism).
  • This test measures the amount of total thyroxine (T4) in your blood. T4 is the main hormone made by your thyroid.
  • Some T4 is attached (bound) to proteins in your blood. Some remains free (free T4). Your health care provider may test you for total T4, free T4, or both.
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