Blood Tests for Hashimoto's Thyroiditis
Primary Diagnostic Tests
The essential blood tests for diagnosing Hashimoto's thyroiditis are TSH, free T4, anti-thyroid peroxidase (anti-TPO) antibodies, and anti-thyroglobulin (anti-Tg) antibodies. 1
Core Laboratory Panel
TSH (Thyroid-Stimulating Hormone): This is the most sensitive screening test for thyroid dysfunction, with sensitivity above 98% and specificity greater than 92% 2. TSH will be elevated in hypothyroidism, which is the typical presentation of Hashimoto's thyroiditis 1.
Free T4 (Free Thyroxine): Measure free T4 alongside TSH to distinguish between subclinical hypothyroidism (elevated TSH with normal free T4) and overt hypothyroidism (elevated TSH with low free T4) 2. This distinction is critical for treatment decisions 2.
Anti-TPO Antibodies: These are the hallmark serologic marker of Hashimoto's thyroiditis and are present in the majority of patients with the disease 1. The presence of anti-TPO antibodies confirms autoimmune etiology and predicts a higher risk of progression to overt hypothyroidism (4.3% per year versus 2.6% in antibody-negative individuals) 2, 3.
Anti-Thyroglobulin (Anti-Tg) Antibodies: These antibodies are also characteristic of Hashimoto's thyroiditis 1. Elevated anti-Tg levels are significantly associated with symptom burden in Hashimoto's patients, including symptoms like fragile hair, face edema, edema of the eyes, and harsh voice 4.
Interpretation Algorithm
When TSH is Elevated (>4.5 mIU/L):
If free T4 is low: This indicates overt hypothyroidism, and levothyroxine therapy should be initiated immediately 2.
If free T4 is normal: This indicates subclinical hypothyroidism 2. For TSH >10 mIU/L, initiate levothyroxine therapy regardless of symptoms 2. For TSH 4.5-10 mIU/L, treatment decisions should be individualized based on symptoms, pregnancy status, or presence of anti-TPO antibodies 2.
When Antibodies are Positive:
Positive anti-TPO and/or anti-Tg antibodies with elevated TSH: This confirms the diagnosis of Hashimoto's thyroiditis as the cause of hypothyroidism 1, 5.
Positive antibodies with normal TSH and free T4: These patients have autoimmune thyroiditis but are currently euthyroid 3. They should be monitored with repeat thyroid function tests every 3-6 months, as they have increased risk of developing hypothyroidism 3.
Additional Considerations
Confirm elevated TSH with repeat testing after 3-6 weeks, as 30-60% of elevated TSH levels normalize spontaneously 2. Do not initiate treatment based on a single abnormal value 2.
Free T3 measurement is generally not necessary for the initial diagnosis of Hashimoto's thyroiditis, as it does not add significant diagnostic information in most cases 2. However, it may be considered if TSH and free T4 results are discordant or if symptoms persist despite normal TSH and free T4 3.
Common Diagnostic Pitfalls
Missing the diagnosis by only checking TSH: Always measure both TSH and free T4 together to properly classify the degree of thyroid dysfunction 2.
Failing to check thyroid antibodies: Without antibody testing, you cannot confirm the autoimmune etiology of hypothyroidism, which has implications for prognosis and monitoring 1, 5.
Treating based on antibody positivity alone: Positive antibodies without thyroid dysfunction (normal TSH and free T4) do not require immediate treatment, only monitoring 3.
Overlooking anti-Tg antibodies: While anti-TPO antibodies are more commonly measured, anti-Tg antibodies provide additional diagnostic value and correlate with symptom burden 4. Screen for both antibodies in patients with significant symptoms 4.