What antibodies should be checked for the diagnosis of hypothyroidism?

Antibody Testing for Hypothyroidism

Check anti-thyroid peroxidase (anti-TPO) antibodies first, as they are the strongest predictor of autoimmune hypothyroidism, and add anti-thyroglobulin (anti-Tg) antibodies to increase diagnostic sensitivity, especially when anti-TPO is negative but clinical suspicion remains high. 1

Primary Antibody Testing Algorithm

First-Line Testing

• Anti-TPO antibodies are the most important initial test because they are more predictive of Hashimoto's thyroiditis than anti-thyroglobulin antibodies in multivariate analysis 1
• TPO antibodies identify autoimmune etiology and are the strongest predictor of progression to overt hypothyroidism 2
• In patients with spontaneous hypothyroidism, >99% are antibody-positive (TPOAb or TgAb), with TPOAb measurable more often than TgAb (95.9% vs. 80.7%) 3

Second-Line Testing

• Add anti-thyroglobulin (anti-Tg) antibodies when anti-TPO is negative but clinical suspicion for autoimmune thyroid disease remains high 1
• The combination of both antibodies increases diagnostic sensitivity significantly—in Hashimoto's thyroiditis, TgAb positivity reaches 96.4% compared to only 44.6% with older hemagglutination techniques 4
• Testing both antibodies together is particularly valuable because approximately 10-25% of confirmed autoimmune thyroid disease cases may have negative TPO antibodies alone 1

Timing of Antibody Testing

Optimal Testing Conditions

• Perform antibody testing when the patient is clinically stable, shortly after diagnosis of hypothyroidism to confirm autoimmune etiology 1
• Avoid testing during acute illness, hyperglycemia, ketosis, weight loss, or other metabolic disturbances because thyroid function tests may be misleading due to euthyroid sick syndrome 1, 5
• If tests are performed during metabolic instability, repeat them after achieving metabolic stability 5

Morning Collection

• Morning testing is preferred for more accurate results, especially when assessing multiple endocrine functions simultaneously 5
• For serial measurements in monitoring, collect samples at approximately the same time of day for consistency 5

Clinical Interpretation and Risk Stratification

Prognostic Value

• Patients with positive thyroid antibodies have a 4.3% per year risk of developing overt hypothyroidism versus 2.6% per year in antibody-negative individuals 2
• The presence of antibodies predicts future thyroid dysfunction, with hypothyroidism being the most common evolution 1
• Even with normal thyroid function tests, elevated antibodies represent early-stage autoimmune thyroid disease, most commonly Hashimoto's thyroiditis 2

Association with Thyroid Pathology

• Both TPOAb and TgAb are positively associated with thyroid enlargement in multivariate analysis 3
• High antibody titers can be highly suggestive of autoimmune thyroid disease, though they may be absent in up to 10-25% of confirmed cases 1

Special Population Considerations

Children with Type 1 Diabetes

• Test for anti-TPO and anti-Tg antibodies shortly after diagnosis of type 1 diabetes, as approximately 25% have thyroid autoantibodies at diagnosis 2, 1
• Ensure testing is performed when clinically stable or after glycemic control has been established 5

Patients with Other Autoimmune Diseases

• Consider early antibody detection in patients with other autoimmune conditions due to increased risk of thyroid autoimmunity 1
• Screen for associated conditions including type 1 diabetes, celiac disease, pernicious anemia, and primary adrenal insufficiency 2

Common Pitfalls to Avoid

Laboratory Variability

• Different laboratory platforms and assays produce varying results, making direct comparison of values across laboratories problematic 2
• Use the same laboratory for serial measurements when monitoring disease progression 2

False Positives in Older Assays

• Anti-thyroglobulin antibodies can produce false positive results in older antimicrosome antibody (AMC) tests 6
• Modern radioassays for TPOAb and TgAb are more specific and sensitive than older hemagglutination techniques 4

Overdiagnosis Concerns

• Many individuals with mildly elevated antibodies may never progress to overt thyroid dysfunction 2
• Labeling asymptomatic patients may have adverse psychological consequences 2
• Focus on clinical context and thyroid function tests (TSH, free T4) in addition to antibody status 2

Follow-Up Monitoring Strategy

Antibody-Positive Patients with Normal Thyroid Function

• Measure TSH every 6-12 months to monitor for progression to hypothyroidism 2, 1
• Increase monitoring frequency to every 6 months if TSH is trending upward or symptoms develop 2
• Educate patients about symptoms of hypothyroidism (unexplained fatigue, weight gain, hair loss, cold intolerance, constipation, depression) for early detection 2

Treatment Thresholds

• Initiate levothyroxine when TSH rises above 10 mIU/L or if symptoms of hypothyroidism develop 2
• Current guidelines do not recommend treatment with levothyroxine for normal thyroid function with positive antibodies alone 2