What lab values are used to diagnose hypothyroidism?

Lab Values for Diagnosing Hypothyroidism

The primary lab test for diagnosing hypothyroidism is thyroid-stimulating hormone (TSH) with elevated levels above 4.5 mIU/L, along with low or normal free thyroxine (T4) levels. 1

Primary Diagnostic Tests

• TSH (Thyroid-Stimulating Hormone):

  • First-line screening test for hypothyroidism 2
  • Reference range: 0.45-4.5 mIU/L 1
  • Elevated TSH (>4.5 mIU/L) with normal free T4 indicates subclinical hypothyroidism
  • Elevated TSH with low free T4 indicates overt hypothyroidism

• Free T4 (Thyroxine):

  • Second test to perform when TSH is abnormal 2
  • Low free T4 with high TSH confirms primary hypothyroidism
  • Normal free T4 with high TSH indicates subclinical hypothyroidism

Additional Testing

• Thyroid Peroxidase (TPO) Antibodies:

  • Useful when hypothyroidism is confirmed to identify autoimmune etiology (Hashimoto's thyroiditis) 1
  • Positive TPO antibodies suggest autoimmune thyroid disease

• Free T3 (Triiodothyronine):

  • Generally not needed for initial diagnosis of hypothyroidism
  • May be measured if TSH is undetectable and free T4 is normal to rule out T3 thyrotoxicosis 2

Special Situations

Secondary (Central) Hypothyroidism

• Low free T4 with low or inappropriately normal TSH 3
• Additional pituitary hormone testing may be needed

Pregnancy

• TSH reference ranges differ during pregnancy 1
• TSH and free T4 should be measured at minimum during each trimester
• Maintain TSH in trimester-specific reference range

Subclinical Hypothyroidism

• Defined as TSH above reference range (>4.5 mIU/L) with normal free T4 1
• Further classified as mild (TSH 4.5-10.0 mIU/L) or severe (TSH >10.0 mIU/L) 1

Monitoring Thyroid Function

• Adults with Primary Hypothyroidism:

  • Monitor TSH 6-8 weeks after any dosage change 4
  • Once stable, evaluate every 6-12 months 4

• Pediatric Patients:

  • Monitor TSH and total or free T4 at 2 and 4 weeks after treatment initiation
  • Then 2 weeks after any dosage change
  • Every 3-12 months after dosage stabilization 4

Common Pitfalls

1. False TSH elevations can occur due to:

   • Recovery from severe illness
   • Recovery from thyroiditis
   • Untreated adrenal insufficiency
   • Heterophilic antibodies in some assays 1

2. Timing of testing:

   • For suspected hypophysitis, tests should be conducted in the morning around 8 am 1
   • TSH has diurnal variation

3. Non-thyroidal illness can affect thyroid function tests, particularly in hospitalized patients 1

4. Medication interference:

   • Glucocorticoids and dopamine can suppress TSH levels 1
   • Recent iodinated contrast can affect uptake scans

Remember that laboratory values must be interpreted in the clinical context, as symptoms of hypothyroidism (fatigue, weight gain, cold intolerance, constipation, dry skin) are often nonspecific 5. The combination of appropriate laboratory testing and clinical evaluation provides the most accurate diagnosis.