which blood tests diagnose an underactive thyroid

An underactive thyroid (hypothyroidism) is primarily diagnosed using blood tests that measure Thyroid-Stimulating Hormone (TSH) and Free Thyroxine (Free T4).¹²

Main Thyroid Blood Tests

• TSH (Thyroid-Stimulating Hormone): This is the main "first-line" test ordered by doctors. TSH is a messenger produced by your pituitary gland in the brain. If your thyroid gland is not making enough hormones, your brain releases more TSH to push it to work harder. A high TSH level is the primary indicator of an underactive thyroid.¹²³⁴
• Free T4 (Thyroxine): T4 is the main hormone produced directly by the thyroid gland. Measuring "Free" T4 checks the portion of the hormone that is not bound to proteins and is actively available for your body to use. In overt hypothyroidism, a high TSH is paired with a low Free T4 level.¹²³
• Free T3 (Triiodothyronine): T3 is the active hormone created when the body converts T4. While not always part of a routine initial screen, doctors may check Free T3 if conversion issues are suspected.¹²³
• Thyroid Antibody Tests (TPOAb and TgAb): These tests check for thyroid peroxidase antibodies or thyroglobulin antibodies. They help determine if an autoimmune condition like Hashimoto’s disease is the root cause of the underactive thyroid.¹²³

You can read more about standard screening guidelines through the NHS Underactive Thyroid Guide or review diagnostic overviews from the NIDDK Thyroid Tests Page.¹²If you'd like, tell me:

• Are you experiencing specific symptoms (like fatigue, weight gain, or feeling cold)?
• Have you already had lab results back from a doctor?

I can help explain what those specific markers mean.

The primary blood test used to diagnose an underactive thyroid (hypothyroidism) is a Thyroid-Stimulating Hormone (TSH) test, often paired with a Free T4 (thyroxine) test.¹²

Key Blood Tests for Hypothyroidism

• TSH (Thyroid-Stimulating Hormone): This is usually the first test ordered. TSH is produced by the pituitary gland; if your thyroid is underactive and not making enough hormones, your pituitary pumps out more TSH to try to kickstart it, resulting in a high TSH level.¹²
• Free T4 (Free Thyroxine): This measures the main active hormone produced by the thyroid gland. In an underactive thyroid, Free T4 levels are typically low alongside a high TSH.¹
• Free T3 (Free Triiodothyronine) / Total T3: This measures another thyroid hormone. It is less commonly needed for diagnosing standard hypothyroidism, as TSH and Free T4 are usually sufficient.¹
• Thyroid Antibody Tests (e.g., TPOAb - Thyroid Peroxidase Antibodies): If an elevated TSH confirms hypothyroidism, doctors often test for antibodies to see if the cause is Hashimoto’s disease, an autoimmune condition.¹

What the Results Mean

You can learn more about general evaluation procedures on the British Thyroid Foundation.

If you are experiencing symptoms, sharing what specific symptoms you have or whether you have already had initial blood work done can help me give you more relevant context.

The main blood tests used to diagnose an underactive thyroid (hypothyroidism) are:

1. Thyroid-stimulating hormone (TSH)

This is the most important initial test.

• High TSH: Usually suggests an underactive thyroid.
• Normal TSH: Usually indicates normal thyroid function.
• Very high TSH: Can indicate more significant hypothyroidism.

2. Free T4 (FT4)

Measures the amount of circulating thyroxine, a thyroid hormone.

• Low FT4 + high TSH: Usually confirms primary hypothyroidism.
• Normal FT4 + high TSH: May indicate subclinical hypothyroidism.

3. Thyroid peroxidase antibodies (TPOAb)

Checks for antibodies that attack the thyroid gland.

• Positive TPO antibodies can indicate Hashimoto's thyroiditis, an autoimmune condition and a common cause of an underactive thyroid.
• They can be positive even when thyroid hormone levels are still normal.

Other tests sometimes considered

What should you ask your GP for?

In the UK, I'd suggest asking for:

1. TSH
2. Free T4 (FT4), particularly if TSH is abnormal
3. TPO antibodies if TSH is raised

These are generally sufficient to establish whether you have an underactive thyroid and whether it may be autoimmune in origin.

One important point: Symptoms of an underactive thyroid can include:

• Persistent tiredness
• Weight gain
• Feeling cold
• Hair thinning or hair loss
• Brain fog and poor concentration
• Constipation
• Dry skin
• Low mood
• Heavy or irregular periods

You don't need to have all these symptoms to have hypothyroidism.

If you're experiencing some of these symptoms, I can also explain what thyroid blood test results (TSH and FT4) mean and what levels are considered abnormal.