What blood tests are used to diagnose hypothyroidism and explore its causes?

Blood Tests for Diagnosing Hypothyroidism and Exploring Its Causes

The primary blood test for diagnosing hypothyroidism is serum TSH (thyroid-stimulating hormone), which should be followed by Free T4 testing when TSH is abnormal to confirm the diagnosis and determine its severity. 1, 2

Diagnostic Testing Algorithm

Initial Testing

• TSH (Thyroid-Stimulating Hormone)

  • First-line screening test for thyroid dysfunction
  • Elevated TSH indicates primary hypothyroidism
  • Normal range: 0.4-4.0 mIU/L 3

• Free T4 (Thyroxine)

  • Should be measured when TSH is abnormal
  • Low Free T4 with elevated TSH confirms overt hypothyroidism
  • Normal Free T4 with elevated TSH indicates subclinical hypothyroidism 1, 2

Additional Testing to Determine Cause

• Thyroid Antibodies
  • TPO (Thyroid Peroxidase) Antibodies: Primary marker for Hashimoto's thyroiditis, the most common cause of hypothyroidism in industrialized nations 4, 1
  • TGA (Thyroglobulin Antibodies): Additional marker for autoimmune thyroiditis

Classification Based on Test Results

1. Primary Hypothyroidism:

   • Elevated TSH + Low Free T4 = Overt hypothyroidism
   • Elevated TSH + Normal Free T4 = Subclinical hypothyroidism 5

2. Secondary (Central) Hypothyroidism:

   • Low/Normal TSH + Low Free T4 = Central hypothyroidism (pituitary or hypothalamic dysfunction) 5

Causes of Hypothyroidism and Related Testing

Common Causes and Associated Tests

1. Chronic Autoimmune Thyroiditis (Hashimoto's Disease)

   • Most common cause in industrialized nations 4
   • Diagnostic markers: Elevated TPO antibodies, TGA antibodies

2. Iodine Deficiency

   • Most common cause worldwide 4
   • Testing: Urinary iodine concentration

3. Post-Treatment Hypothyroidism

   • Following radioactive iodine therapy or thyroidectomy
   • Testing: Medical history review, surgical history

4. Medication-Induced Hypothyroidism

   • Caused by amiodarone, lithium, tyrosine kinase inhibitors
   • Testing: Medication review, timing of symptom onset relative to medication initiation

5. Subacute Thyroiditis

   • Testing: ESR, CRP (inflammatory markers)

Monitoring Protocol

• For newly diagnosed patients, repeat TSH and Free T4 testing 6-8 weeks after initiating levothyroxine therapy 6
• For stable patients on appropriate replacement dosage, evaluate clinical and biochemical response every 6-12 months 6
• Multiple tests should be done over a 3-6 month interval to confirm abnormal findings 4

Special Considerations

• Pregnancy: TSH and Free T4 should be measured as soon as pregnancy is confirmed and during each trimester, with trimester-specific reference ranges 6
• Secondary Hypothyroidism: Cannot rely on TSH for monitoring; must use Free T4 levels 2
• Elderly Patients: More likely to progress from subclinical to overt hypothyroidism and require more aggressive monitoring 1

Pitfalls to Avoid

• Relying solely on TSH without confirming with Free T4 when TSH is abnormal
• Failing to test for antibodies when hypothyroidism is detected
• Not considering medication effects on thyroid function
• Using inappropriate reference ranges (especially during pregnancy)
• Testing too soon after dose adjustments (before steady state is achieved)

By following this systematic approach to blood testing, clinicians can accurately diagnose hypothyroidism and identify its underlying cause, leading to appropriate treatment and improved patient outcomes.