Diagnostic Criteria for Hashimoto's Thyroiditis
The diagnosis of Hashimoto's thyroiditis is established through laboratory confirmation of elevated TSH (with or without low free T4) combined with positive thyroid autoantibodies (anti-TPO and/or anti-thyroglobulin), without requiring imaging or tissue biopsy in most cases. 1, 2
Laboratory Testing Algorithm
Initial Screening
- Measure serum TSH as the first-line test to assess thyroid function status 1, 2
- Obtain free T4 levels to differentiate subclinical hypothyroidism (elevated TSH with normal free T4) from overt hypothyroidism (elevated TSH with low free T4) 1, 2
- Test for anti-thyroid peroxidase (anti-TPO) antibodies and anti-thyroglobulin (anti-Tg) antibodies as the serologic markers of autoimmune thyroid disease 1, 3, 4, 5
The presence of anti-TPO antibodies has an area under the curve of 0.67, while anti-Tg antibodies have an AUC of 0.63 as individual predictors 3. When combined, these antibodies provide stronger diagnostic accuracy 3.
Confirmatory Approach
- Repeat abnormal TSH findings over 3-6 months before confirming persistent thyroid dysfunction, as transient elevations can occur 2
- Recognize that normal IgG levels do not exclude the diagnosis of Hashimoto's thyroiditis 1
- Be aware that non-thyroid illness can cause false-positive TSH results, particularly in hospitalized or acutely ill patients 2
Clinical Presentation to Recognize
Look specifically for:
- Fatigue, weight gain, cold intolerance, constipation, hair loss, and voice changes as the characteristic symptom constellation 1
- Palpable thyroid abnormalities on physical examination, which may include diffuse goiter or thyroid atrophy 2, 4
- Female sex and advancing age, as women over 60 years have the highest prevalence 2
- Family history of thyroid disease or other autoimmune disorders (type 1 diabetes, celiac disease, vitiligo, alopecia) 1, 2, 6
Role of Imaging
Imaging is NOT required for diagnosis when laboratory tests are definitive 7, 1. Specifically:
- No role for ultrasound, CT, MRI, FDG-PET/CT, or radionuclide scans in the routine diagnostic workup of hypothyroidism in adults 7, 1
- Thyroid ultrasound may be performed if nodules are palpable or suspected, to assess for features requiring fine-needle aspiration (microcalcifications, irregular borders, suspicious characteristics) 2
- Reserve fine-needle aspiration for nodules that meet size and ultrasound criteria for malignancy risk, not for diagnosing Hashimoto's thyroiditis itself 1, 2
Diagnostic Pitfalls to Avoid
- Do not rely on TSH alone without measuring free T4 and thyroid antibodies 2
- Do not perform routine calcitonin screening unless medullary thyroid carcinoma is specifically suspected 2
- Do not assume imaging is necessary for diagnosis—this is a laboratory-based diagnosis 7, 1
- Do not exclude the diagnosis based on absence of symptoms, as 25-34% of patients with autoimmune thyroid disease are asymptomatic at presentation 7
- Recognize that the presence of pruritus or hyperpigmentation is inconsistent with typical Hashimoto's thyroiditis and suggests alternative diagnoses 7
Multi-Criteria Scoring System (When Diagnosis is Uncertain)
When clinical judgment is difficult, a comprehensive scoring system combining anti-TPO antibodies, anti-Tg antibodies, and thyroid vascularity on ultrasound achieves an AUC of 0.72, with sensitivity of 45.5% and specificity of 89.0% at a cutoff score of 1.7 3. This approach identifies individuals at higher risk for developing subsequent hypothyroidism (OR 2.30) 3.