Diagnosis of Hashimoto's Thyroiditis
To diagnose Hashimoto's thyroiditis, you need elevated TSH (or normal TSH in early disease), positive anti-thyroid peroxidase (anti-TPO) antibodies, and ideally thyroid ultrasound showing heterogeneous hypoechoic pattern—though imaging is not mandatory when laboratory findings are definitive.
Essential Diagnostic Tests
Primary Laboratory Assessment
Measure serum TSH first as the initial screening test for thyroid dysfunction, which has approximately 98% sensitivity and 92% specificity for detecting thyroid disease 1, 2.
Obtain free T4 levels to distinguish between subclinical hypothyroidism (elevated TSH with normal free T4) and overt hypothyroidism (elevated TSH with low free T4) 1, 2.
Check anti-thyroid peroxidase (anti-TPO) antibodies, which are positive in 73.5–96.4% of histologically confirmed Hashimoto's thyroiditis cases 2, 3, 4.
Consider anti-thyroglobulin (anti-Tg) antibodies if anti-TPO is negative, as anti-Tg can be positive in up to 96.4% of cases when measured by sensitive radioassay methods, providing additional diagnostic accuracy 2, 3.
Thyroid Imaging
Thyroid ultrasound reveals heterogeneous hypoechoic echotexture characteristic of Hashimoto's thyroiditis and helps identify any nodules requiring further evaluation 2, 4, 5.
Imaging is NOT required for diagnosis when laboratory tests (elevated TSH and positive anti-TPO antibodies) are definitive—the American College of Radiology states that ultrasound, CT, MRI, or radionuclide scans have no role in routine diagnostic workup of hypothyroidism 2.
Reserve ultrasound for evaluating palpable thyroid nodules or when nodules have suspicious features requiring exclusion of malignancy 2.
Clinical Context for Testing
High-Risk Populations Requiring Screening
Women over 60 years have the highest prevalence of Hashimoto's thyroiditis and should be tested when symptomatic 2.
Patients with type 1 diabetes mellitus are at significantly higher risk and warrant screening 2.
Individuals with family history of thyroid disease or other autoimmune disorders should undergo testing 2.
Patients presenting with symptoms of hypothyroidism—fatigue, weight gain, cold intolerance, constipation, dry skin, or cognitive slowing—require TSH and antibody testing 1, 2, 6.
Individuals with palpable thyroid abnormalities on physical examination need diagnostic workup 2.
Important Clinical Caveat
- 25–38% of patients with Hashimoto's thyroiditis are completely asymptomatic at presentation, so absence of symptoms does not exclude the diagnosis 2, 6.
Diagnostic Algorithm
Step 1: Initial Laboratory Testing
- Measure TSH and free T4 simultaneously
- If TSH is elevated (>4.5 mIU/L) or if clinical suspicion is high despite normal TSH, proceed to Step 2
Step 2: Antibody Testing
- Check anti-TPO antibodies (most sensitive single test)
- If anti-TPO is negative but clinical suspicion remains high, add anti-thyroglobulin antibodies
- Positive antibodies in the setting of elevated TSH confirm Hashimoto's thyroiditis
Step 3: Confirmatory Testing (When Needed)
Repeat abnormal TSH findings after 3–6 months before confirming persistent thyroid dysfunction, as 30–60% of transient TSH elevations normalize spontaneously 1, 2.
Thyroid ultrasound is optional but can provide supportive evidence when diagnosis is uncertain or when evaluating for coexisting nodules 2, 5.
Fine-needle aspiration (FNA) is reserved exclusively for thyroid nodules meeting size and ultrasound criteria for malignancy risk (microcalcifications, irregular borders, increased vascularity) 2.
Diagnostic Accuracy by Test Combination
Rule-of-Thumb Method
If two or more of five diagnostic markers are positive (thyroid antibodies, elevated TSH, characteristic ultrasound findings, abnormal thyroid scan, positive perchlorate discharge test), Hashimoto's thyroiditis is the likely diagnosis with 67% accuracy by simple criteria or 88% accuracy by computer-assisted methods 7.
Testing four or preferably all five criteria yields the best diagnostic accuracy, though in modern practice TSH, free T4, anti-TPO antibodies, and ultrasound constitute the core workup 7.
Antibody Testing Performance
Anti-TPO antibodies detected by radioassay show 73.5% sensitivity for histologically proven Hashimoto's thyroiditis 3.
Anti-thyroglobulin antibodies by radioassay demonstrate 96.4% sensitivity, making them particularly valuable when anti-TPO is negative 3.
Traditional hemagglutination methods (TGHA, MCHA) are less sensitive (44.6% and 65.1% respectively) compared to modern radioassay techniques 3.
Common Diagnostic Pitfalls
Do not rely on TSH alone—antibody testing is essential to confirm autoimmune etiology and distinguish Hashimoto's from other causes of hypothyroidism 2.
Avoid routine calcitonin screening in patients with thyroid nodules unless medullary thyroid carcinoma is specifically suspected 2.
Recognize that non-thyroidal illness can cause false-positive TSH elevations, particularly in hospitalized or acutely ill patients—repeat testing after recovery 2.
Do not exclude diagnosis based on normal thyroid size—Hashimoto's thyroiditis can present with normal-sized, enlarged, or atrophic glands depending on disease stage 4.
Approximately 25% of positive diagnoses may be false positives, nearly all occurring in patients with goiter associated with defects in thyroid hormone synthesis 7.
When Biopsy Is Indicated
Fine-needle aspiration cytology combined with ultrasound, thyroid function tests, and antibody measurements can detect hypothyroid, subclinical hypothyroid, or euthyroid states of Hashimoto's thyroiditis and guide appropriate therapy 6.
Cytological grading of Hashimoto's thyroiditis provides additional diagnostic information when combined with other modalities, particularly in early or atypical presentations 6.
Surgery is performed only when goiter causes significant compression of surrounding cervical structures or when nodule cytology cannot be definitively characterized as benign 4.