Workup for Hashimoto's Thyroiditis
The workup for suspected Hashimoto's thyroiditis should include serum TSH, free T4, and anti-thyroid peroxidase antibodies (TPOAb), with thyroid ultrasound considered when nodular disease is suspected.
Essential Laboratory Tests
The diagnostic approach is straightforward and relies on three key components:
1. Thyroid Function Testing
- Measure serum TSH first - this is the most sensitive screening test for thyroid dysfunction 1, 2
- Add free T4 (not total T4 in non-pregnant patients) to assess the degree of thyroid dysfunction 2
- The normal TSH reference range is 0.45-4.5 mIU/L, though values between 2.5-4.5 mIU/L may identify early hypothyroidism 1
2. Autoantibody Testing
- Anti-thyroid peroxidase antibodies (TPOAb) should be measured when evaluating patients with subclinical hypothyroidism or suspected autoimmune thyroiditis 2
- Anti-thyroglobulin antibodies can also be measured but are less specific 3, 4
- Positive TPOAb confirms autoimmune etiology and predicts progression: 4.3% per year with positive antibodies versus 2.6% per year when negative 2
3. Thyroid Ultrasound
- TPOAb measurement should be considered to identify autoimmune thyroiditis when nodular thyroid disease is suspected 2
- Ultrasound shows characteristic hypoechoic, heterogeneous pattern in Hashimoto's 4, 5
- Important caveat: Obese children may show similar echographic patterns without having Hashimoto's 5
What NOT to Use
Clinical scoring systems, reflex relaxation time, cholesterol levels, and muscle enzymes should not be used to diagnose hypothyroidism 2. These lack diagnostic accuracy and add unnecessary cost without improving clinical decision-making.
Diagnostic Interpretation
The diagnosis is established when you find:
- Elevated TSH (>4.5 mIU/L) with low or low-normal free T4
- Positive TPOAb (markedly elevated, often >1000 IU/mL)
- Characteristic ultrasound findings (if performed)
A practical rule: If two or more diagnostic markers are positive (elevated TSH, positive TPOAb, characteristic ultrasound, goiter on exam, or abnormal thyroid scan), Hashimoto's is the likely diagnosis 6.
Clinical Presentation Considerations
Thyroid function at presentation varies widely 5:
- Some patients present with overt hypothyroidism (high TSH, low free T4)
- Others have subclinical hypothyroidism (high TSH, normal free T4)
- A minority may have transient hyperthyroidism from thyroid destruction
- Some remain euthyroid with only positive antibodies
Associated Conditions to Screen For
Check for other autoimmune diseases, as Hashimoto's frequently coexists with 5:
- Type 1 diabetes
- Celiac disease
- Vitiligo
- Alopecia
Special Populations
In pregnancy, measure total T4 or free T4 index plus TSH, using trimester-specific reference ranges 2. In hospitalized patients, only measure TSH if there is specific suspicion for thyroid dysfunction to avoid false positives from nonthyroidal illness 2.