Hyperthyroidism blood test with TSI (TRAb)

Blood test to investigate possible hyperthyroidism and point towards its cause, especially when a previous test has shown a low TSH. It includes TSH, free T4, free T3 and thyroid antibodies, among them TRAb or TSI, which help identify Graves' disease and tell it apart from other causes.

No prescription or appointment needed

Results in 1-2 working days

Includes: TSH · Free T4 · Free T3 · Thyroid peroxidase antibodies · TSH receptor antibodies. See what each one measures

What it is for

It answers two questions. The first is whether a low TSH comes with an excess of thyroid hormones, which would mean overt hyperthyroidism, or whether free T4 and free T3 are still within the range.

The second is whether the antibodies typical of Graves' disease, an autoimmune cause of hyperthyroidism, are present. Bear in mind that TSH receptor antibodies help point towards the diagnosis, but a negative result does not exclude every cause and does not replace a medical assessment.

When it is recommended

A low TSH, especially if a previous blood test also showed raised T4 or T3
Symptoms of hyperthyroidism: palpitations, tremor, nervousness, sweating or feeling the heat
Weight loss while appetite is normal or increased and food intake is adequate
Eye discomfort or a feeling that the eyes are more prominent
Goitre (an enlarged thyroid) together with symptoms of an overactive thyroid

Clinical comment

When TSH is low, the first step is to find out whether free T4 or free T3 are raised. That distinction separates overt hyperthyroidism from other situations that can also lower TSH, and avoids drawing conclusions from a single figure.

If excess hormone is confirmed, knowing the cause matters because it determines the next tests and the treatment. A positive TRAb or TSI result, when it fits with the rest of the assessment, strongly supports a diagnosis of Graves' disease.

Free T3 has a specific role here: it can reveal T3 thyrotoxicosis, with raised T3 while free T4 is still normal.

This blood test does not settle the causes that require imaging. If the antibodies are negative or the results do not explain the picture, the medical assessment may require an ultrasound, a thyroid scan or other tests depending on the case.

Dr. Antonio Mas

What is tested

TSH: The hormone the pituitary gland uses to regulate the thyroid. A low TSH can be the first sign of hyperthyroidism, but it has to be assessed together with the thyroid hormones.
Free T4: The main hormone produced by the thyroid, measured in its free fraction. It shows whether there is an excess of circulating thyroxine.
Free T3: The thyroid hormone with the greatest activity in the tissues. In some cases of hyperthyroidism it rises before free T4.
Thyroid peroxidase antibodies (anti-TPO): Antibodies against thyroid peroxidase. They indicate a tendency to thyroid autoimmunity, although on their own they do not distinguish between its different forms.
TSH receptor antibodies (TRAb): They detect antibodies directed against the TSH receptor. The stimulating ones activate the gland and are characteristic of Graves' disease.

How to prepare

No fasting is required. 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

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 recommend measuring free T4 and free T3 when TSH is low. They also include TSH receptor antibodies among the tests that help establish the cause of hyperthyroidism and identify Graves' disease.

American Thyroid Association · Thyroid 26(10):1343-1421
2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis
When the cause of hyperthyroidism is not clear from the symptoms and the initial blood tests, lists TSH receptor antibodies among the tests indicated to establish it.

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.