Hashimoto's Thyroiditis: Blood Tests You Need

Hashimoto's thyroiditis is the most common cause of hypothyroidism in the United Kingdom, yet it remains remarkably under-investigated by routine NHS blood tests. It is an autoimmune condition in which the immune system produces antibodies that gradually destroy the thyroid gland. Over months or years, this destruction leads to falling thyroid hormone levels, a constellation of symptoms including fatigue, weight gain, cold intolerance, brain fog, hair thinning, and low mood, and eventually a formal diagnosis of hypothyroidism.

The problem is that most GP practices only measure thyroid stimulating hormone (TSH) when assessing thyroid function. TSH is a useful screening tool, but it tells you almost nothing about autoimmune thyroid disease in its early stages. Understanding which blood tests you actually need, and why, can make the difference between catching Hashimoto's early and waiting years for a diagnosis you could have had much sooner.

How Hashimoto's Damages the Thyroid

In Hashimoto's thyroiditis, the immune system produces autoantibodies — primarily anti-thyroid peroxidase (anti-TPO) and anti-thyroglobulin (anti-Tg) — that target proteins within the thyroid gland. Thyroid peroxidase is an enzyme essential for producing thyroid hormones. Thyroglobulin is a protein used as a building block for those hormones. When antibodies attack these proteins, the thyroid becomes chronically inflamed and is progressively destroyed.

This destruction does not happen overnight. Many people with Hashimoto's have elevated antibodies for years before their TSH rises above the reference range. During this subclinical phase, they may experience symptoms that are dismissed as stress, ageing, or depression. The thyroid may still produce enough hormone to keep TSH within its broad "normal" range, but the autoimmune process is already well underway.

The Blood Tests You Need

TSH (Thyroid Stimulating Hormone)

TSH is produced by the pituitary gland and acts as a messenger telling the thyroid how much hormone to produce. The standard NHS reference range is 0.27–4.2 mIU/L. When the thyroid starts failing, TSH rises as the pituitary tries to compensate. However, TSH can remain within the normal range for a surprisingly long time in early Hashimoto's. A TSH of 3.8, for example, is technically "normal" but may represent a significant rise for someone whose baseline was 1.2. Context matters.

Free T4 (FT4) and Free T3 (FT3)

FT4 (thyroxine) is the main hormone produced by the thyroid gland, and FT3 (triiodothyronine) is the biologically active form. FT4 is converted into FT3 in peripheral tissues, primarily the liver and kidneys. In Hashimoto's, FT4 may start to drop before TSH moves outside the reference range. FT3 is particularly important because some patients have normal FT4 but low FT3, indicating a conversion problem. This is common in Hashimoto's and can cause persistent symptoms even when TSH and FT4 look acceptable on paper.

Anti-TPO Antibodies

This is the single most important test for diagnosing Hashimoto's thyroiditis. Anti-TPO antibodies are positive in over 90% of people with Hashimoto's. A result above 34 IU/mL is generally considered positive, though reference ranges vary by laboratory. High levels confirm that autoimmune destruction of the thyroid is occurring. Crucially, anti-TPO antibodies can be elevated for years before any change in TSH or thyroid hormones. Testing for them is the earliest way to detect Hashimoto's.

Anti-Thyroglobulin Antibodies (Anti-Tg)

Anti-Tg antibodies are positive in approximately 60–80% of Hashimoto's patients. They are less sensitive than anti-TPO but still clinically significant. In a small percentage of Hashimoto's cases, anti-TPO may be negative while anti-Tg is positive. Testing both antibodies provides the most complete picture.

Beyond the Thyroid Panel: Essential Supporting Tests

Hashimoto's does not exist in isolation. It frequently co-occurs with nutrient deficiencies, partly because autoimmune conditions are associated with gut inflammation and malabsorption, and partly because certain nutrients are directly involved in thyroid function.

Vitamin D

Vitamin D deficiency is remarkably common in people with autoimmune thyroid disease. Research published in the journal Thyroid found that patients with Hashimoto's had significantly lower vitamin D levels compared to healthy controls. In the United Kingdom, where roughly 1 in 5 adults has vitamin D levels below 25 nmol/L (the deficiency threshold), people with Hashimoto's are at even greater risk. Optimal levels for immune regulation are generally considered to be 75–100 nmol/L.

Vitamin B12

Hashimoto's thyroiditis is associated with an increased prevalence of pernicious anaemia and other causes of B12 deficiency. Symptoms of B12 deficiency — fatigue, cognitive difficulties, tingling in hands and feet — overlap significantly with hypothyroid symptoms, making it easy to attribute everything to the thyroid when a separate, treatable deficiency is also contributing. The normal range is typically 200–900 ng/L, but many functional medicine practitioners consider levels below 500 ng/L suboptimal.

Iron and Ferritin

Iron is essential for thyroid hormone production, and ferritin (the storage form of iron) is needed for the conversion of T4 to T3. People with Hashimoto's frequently have low ferritin, partly due to malabsorption and partly because hypothyroidism itself can reduce stomach acid production, impairing iron absorption. A ferritin level below 30 µg/L is considered depleted, even if technically within some laboratory reference ranges. Optimal ferritin for thyroid function is generally considered to be above 70 µg/L.

Selenium

Selenium plays a critical role in thyroid hormone metabolism. The enzyme that converts T4 to the active T3 (type 1 deiodinase) is a selenoprotein. Multiple studies have shown that selenium supplementation can reduce anti-TPO antibody levels in Hashimoto's patients. The UK Reference Nutrient Intake is 75 µg/day for men and 60 µg/day for women, but many people fall short. While selenium is not included in standard NHS blood panels, understanding your thyroid antibody and T3 levels can help guide decisions about supplementation.

Why Your GP Probably Is Not Testing All of This

NHS guidelines for thyroid testing are deliberately streamlined. In most cases, GPs are advised to test TSH first. If TSH is abnormal, FT4 is added. FT3 and antibody testing are not routinely performed in primary care. This approach makes sense from a population screening perspective — TSH catches the majority of overt thyroid dysfunction — but it misses the early autoimmune phase of Hashimoto's entirely.

This means that if you have a family history of thyroid disease, are experiencing symptoms consistent with hypothyroidism, or have another autoimmune condition (such as type 1 diabetes, coeliac disease, or vitiligo), you may benefit from a more comprehensive panel than what your GP surgery offers.

Testing Options at Lola Health

Our Peak Insights 70 blood test (£185) includes a full thyroid panel with TSH, FT3, FT4, anti-TPO antibodies, and anti-thyroglobulin antibodies, alongside vitamin D, B12, folate, iron, ferritin, and a complete blood count. It is the most comprehensive option for investigating Hashimoto's thyroiditis.

If you are primarily interested in thyroid hormones, our Thyroid & Hormonal Function test (£99) covers TSH, FT3, and FT4. Anti-TPO and anti-Tg are available as add-on biomarkers if you want to screen for autoimmunity without committing to the full 70-biomarker panel.

When to Test and How to Prepare

For the most accurate results, take your blood sample in the morning, ideally before 10am. TSH follows a circadian rhythm and is highest in the early morning, so testing later in the day can produce a falsely reassuring result. If you are already taking levothyroxine, take your sample before your morning dose. Fasting is not strictly required for thyroid tests, but a morning sample on an empty stomach gives the most consistent and comparable results.

If you have already been diagnosed with Hashimoto's and are on treatment, periodic monitoring of antibodies alongside thyroid hormones can help track disease activity and inform treatment adjustments. A declining antibody trend suggests the autoimmune process is quietening. A rising trend may warrant a reassessment of treatment or lifestyle factors.

Key Takeaways

• Hashimoto's thyroiditis is the leading cause of hypothyroidism in the UK, driven by autoimmune destruction of the thyroid gland.
• Anti-TPO antibodies are positive in over 90% of cases and can be elevated years before TSH becomes abnormal.
• A comprehensive panel including TSH, FT3, FT4, anti-TPO, anti-Tg, vitamin D, B12, iron, and ferritin gives the fullest picture.
• Standard GP testing (TSH only) will miss early-stage Hashimoto's in the majority of cases.
• Nutrient deficiencies are common in Hashimoto's and can worsen symptoms independently of thyroid hormone levels.

Understanding your blood results in context — not just whether they fall within a reference range — is the first step towards managing Hashimoto's effectively. If you suspect autoimmune thyroid disease, a comprehensive blood test is the most direct route to answers.