When should thyroid antibodies, such as thyroid-stimulating immunoglobulin (TSI) and thyroperoxidase antibodies (TPOAb), be checked in patients?

When to Check Thyroid Antibodies

Thyroid antibodies should be checked in patients with type 1 diabetes soon after diagnosis, in patients with suspected autoimmune thyroid disease, and when there is unexplained thyroid dysfunction or glycemic variability. 1

Indications for Thyroid Antibody Testing

Type 1 Diabetes Patients

• Test for antithyroid peroxidase (TPOAb) and antithyroglobulin (TgAb) antibodies soon after diagnosis of type 1 diabetes 1
• TPOAb appears to be more predictive than TgAb for future thyroid dysfunction in multivariate analysis 1
• Autoimmune thyroid disease occurs in 17-30% of patients with type 1 diabetes 1
• At diagnosis, approximately 25% of children with type 1 diabetes have thyroid autoantibodies 1

Suspected Graves' Disease

• Check thyroid-stimulating immunoglobulin (TSI) when:
  • Clinical features suggest Graves' disease (hyperthyroidism with diffuse goiter, ophthalmopathy)
  • Differential diagnosis between Graves' disease and other causes of hyperthyroidism is needed 2
  • During pregnancy in women with a history of Graves' disease 3
  • In patients with thyroid eye disease 2

Other Indications

• Unexplained thyroid dysfunction
• Patients with symptoms of autoimmune thyroiditis
• Patients with unexplained glycemic variability in diabetes 1
• First-degree relatives of patients with autoimmune thyroid disease (for screening purposes)
• Patients with other autoimmune disorders (e.g., celiac disease, vitiligo, Addison's disease)

Specific Antibodies and Their Clinical Significance

Thyroid Peroxidase Antibodies (TPOAb)

• Primary antibody used to diagnose Hashimoto's thyroiditis
• Present in 81-98% of Hashimoto's thyroiditis patients (depending on assay) 4
• Predictive of future thyroid dysfunction, particularly hypothyroidism 1
• Should be measured alongside TgAb for comprehensive assessment 3

Thyroglobulin Antibodies (TgAb)

• Complementary to TPOAb testing
• May be positive in some patients who are TPOAb negative 4
• Recent research suggests TgAb may be more prevalent than previously thought in Hashimoto's thyroiditis and painless thyroiditis 4
• Some studies indicate TgAb prevalence (98.6%) may exceed TPOAb prevalence (81.4%) in Hashimoto's thyroiditis 4

Thyroid-Stimulating Immunoglobulin (TSI)

• Specific biomarker for Graves' disease 2
• Recommended for rapid diagnosis of Graves' disease
• Useful for differential diagnosis of hyperthyroidism
• Important during pregnancy for risk assessment of neonatal thyrotoxicosis 2
• Found in 72% of patients with hyperthyroid Graves' disease 5

Follow-up After Initial Testing

• If thyroid antibodies are positive:

  • Measure thyroid-stimulating hormone (TSH) and free T4 at diagnosis when clinically stable or after glycemic control is established 1
  • If normal, recheck every 1-2 years 1
  • Monitor more frequently if the patient develops symptoms of thyroid dysfunction, thyromegaly, abnormal growth rate, or unexplained glycemic variability 1

• If thyroid antibodies are negative:

  • Consider periodic reassessment in high-risk patients (e.g., type 1 diabetes)
  • Recheck if symptoms of thyroid dysfunction develop

Important Clinical Considerations

• Thyroid function tests may be misleading (euthyroid sick syndrome) if performed during acute illness, hyperglycemia, ketosis, or weight loss 1
• If thyroid function tests are abnormal at diagnosis, they should be repeated after metabolic stability is achieved 1
• Subclinical hypothyroidism in diabetic patients may increase risk of symptomatic hypoglycemia and reduce linear growth rate 1
• Hyperthyroidism can alter glucose metabolism and cause deterioration of glycemic control 1

Pitfalls to Avoid

• Don't rely solely on TSH for diagnosis of thyroid dysfunction; both TSH and free T4 should be measured for comprehensive assessment 3
• Don't interpret thyroid function tests in isolation without considering antibody status
• Avoid testing during acute illness, as results may be misleading (euthyroid sick syndrome) 1
• Different commercial kits for antibody testing may yield different results; be aware of laboratory-specific reference ranges 4
• Don't assume TPOAb is always superior to TgAb for diagnosis; recent evidence suggests TgAb may be more prevalent in some populations 4