What is the diagnostic approach for hypothyroidism?

Diagnostic Approach for Hypothyroidism

The primary diagnostic test for hypothyroidism is measuring thyroid-stimulating hormone (TSH), followed by free thyroxine (T4) if TSH is abnormal. This two-step approach provides the most accurate diagnosis of hypothyroidism 1.

Initial Diagnostic Testing

• TSH measurement is the preferred initial test for suspected primary hypothyroidism 1
• If TSH is elevated, free T4 should be measured to differentiate between subclinical and overt hypothyroidism 2
• Testing should be repeated in 2 weeks to 3 months to confirm abnormal results before initiating treatment 2
• Morning testing is preferred for more accurate results, especially when assessing adrenal function simultaneously 2

Interpretation of Results

• Overt primary hypothyroidism: Elevated TSH with low free T4 3
• Subclinical hypothyroidism: Elevated TSH with normal free T4 2
• Central (secondary) hypothyroidism: Low or normal TSH with low free T4 4

Additional Testing to Consider

• Anti-thyroid peroxidase (anti-TPO) antibodies to identify autoimmune etiology (Hashimoto's thyroiditis) 2
  • Presence of antibodies predicts higher risk of developing overt hypothyroidism (4.3% per year vs 2.6% per year in antibody-negative individuals) 2
• Lipid profile, as hypothyroidism can affect cholesterol levels 2
• Assessment for signs and symptoms of hypothyroidism, including:
  • Fatigue (68-83% of patients) 3
  • Weight gain (24-59%) 3
  • Cold intolerance 3
  • Cognitive issues (45-48%) including memory loss and difficulty concentrating 3
  • Menstrual irregularities (approximately 23%) 3
  • Constipation 3

Special Considerations

• No role for imaging in routine diagnosis: Thyroid ultrasound, CT, MRI, or radionuclide scans are not indicated for the initial workup of hypothyroidism 2
• Targeted testing is recommended for high-risk individuals rather than general population screening 3:
  • Patients with type 1 diabetes 2
  • Patients with other autoimmune disorders 2
  • First-degree relatives of patients with autoimmune thyroid disease 3
  • Patients who have received neck radiation or surgery 3
  • Patients taking medications that may affect thyroid function (e.g., amiodarone, immune checkpoint inhibitors) 3

Monitoring After Diagnosis

• For patients on levothyroxine treatment, TSH should be monitored 6-8 weeks after initiating treatment or changing dose 3
• Once stable, annual TSH monitoring is recommended 3
• For central hypothyroidism, free T4 and T3 concentrations should be used for monitoring rather than TSH 1

Common Pitfalls to Avoid

• Euthyroid sick syndrome: Thyroid function tests may be misleading if performed during acute illness, ketosis, or significant weight loss 2
• Pregnancy: TSH levels may increase during pregnancy, requiring more frequent monitoring and potential dose adjustments 5
• Medication interference: Certain medications can affect thyroid function test results 3
• Subclinical hypothyroidism mismanagement: Not all patients with subclinical hypothyroidism require treatment, especially those with TSH between 4.5-10 mIU/L without symptoms 2

Following this diagnostic approach will allow for accurate identification of hypothyroidism and appropriate treatment decisions to prevent complications such as cardiovascular disease, reproductive issues, and in severe cases, myxedema coma 3.