TSH and free T4 blood test (hypothyroidism monitoring)

For follow-up

TSH and free T4 blood test to monitor hypothyroidism that has already been diagnosed and check whether levothyroxine treatment keeps thyroid function within the targets set by your doctor. It is also useful for repeating a TSH that came back abnormal in a previous test.

No prescription or appointment needed

Results in 1-2 working days

Includes: TSH · Free T4. See what each one measures

What it is for

It is useful in two situations. The first is regular monitoring of hypothyroidism that has already been diagnosed, especially when it is treated with levothyroxine. TSH shows the response to treatment and free T4 provides the context needed to assess the result.

The second is to check or confirm a TSH that has come back abnormal for the first time in another blood test. A single figure outside the range is not always enough to make a diagnosis or change a treatment: it has to be interpreted together with free T4, symptoms, medical history and the time the sample was taken.

When it is recommended

Regular monitoring of hypothyroidism that has already been diagnosed, usually once or twice a year
A check after a change in levothyroxine dose, once the time indicated by your doctor has passed
Repeating an abnormal TSH found in a previous blood test
Follow-up of known thyroiditis when only thyroid function needs to be reviewed

Clinical comment

This profile is designed for follow-up. It assumes that the diagnosis and the cause of the hypothyroidism are already known. In primary hypothyroidism treated with levothyroxine, TSH is the main marker and free T4 helps put it in context.

If this is the first time your thyroid is being tested, these two tests may fall short because they do not look for a possible autoimmune cause. In that case the full thyroid blood test, which adds free T3 and anti-TPO antibodies, usually gives more information.

Dr. Antonio Mas

What is tested

TSH: The hormone the pituitary gland uses to regulate the thyroid. In primary hypothyroidism treated with levothyroxine, it is the main reference for monitoring.
Free T4: The fraction of thyroxine that circulates freely and is available to the tissues. It completes the information from TSH and helps assess how much thyroid hormone is present in the blood.

How to prepare

No fasting is required. If you take levothyroxine, have the sample taken before that day's dose, so that the temporary rise in free T4 after taking it does not distort the comparison. If you take biotin supplements, which are common in hair and nail products, stop them 48 hours before, because they can interfere with several thyroid tests.

You can add

Without another needle. It is offered when you order, so you do not have to decide now.

General blood testA general blood test with full blood count, glucose, cholesterol and kidney and liver function: the basic parameters included in any general check-up.+€59.00
General + nutritional blood testEverything in the general blood test (anaemia, kidneys, liver, cholesterol and glucose) plus iron stores, vitamin D, vitamin B12 and folic acid. It is the broadest add-on: it covers the general and the nutritional in one go.+€129.00
Sugar, insulin and inflammationAssesses insulin resistance through fasting insulin and the HOMA-IR index, plus average blood sugar over the last three months and low-grade inflammation.+€59.00

Related blood tests

We have other profiles in Thyroid with more or fewer tests:

The most comprehensive is not always the most useful. If you are not sure, the one we recommend to start with is Full thyroid blood test with anti-TPO antibodies.

What it is based on

Clinical guidelines use TSH as the main marker for monitoring primary hypothyroidism treated with levothyroxine. Free T4 completes the picture when the result needs clarifying or the amount of circulating hormone needs checking. After a dose change, enough time must pass before repeating the test, because TSH takes several weeks to stabilise.

American Thyroid Association · Thyroid 24(12):1670-1751
Guidelines for the Treatment of Hypothyroidism
Recommends TSH as the parameter for monitoring levothyroxine treatment, and waiting four to six weeks after a dose change before measuring it again.

Protocol designed by Dr. Antonio Mas, specialist in Endocrinology and Nutrition, Spanish medical licence no. 080846289, and reviewed in September 2026.

This blood test is a standardised protocol and does not replace a medical consultation. If you have symptoms, a known condition or a complex clinical situation, speak to your doctor.