What is the normal reference range for thyroid‑stimulating hormone (TSH) and how should abnormal TSH results be interpreted and managed?

TSH (Thyroid-Stimulating Hormone): Normal Range and Clinical Management

Normal Reference Range

The normal TSH reference range is 0.45-4.5 mIU/L in adults, though this shifts slightly upward with age (0.4-5.9 mIU/L for ages 70-79 years). 1

• Values below 0.1 mIU/L are considered definitively low 2
• Values above 6.5 mIU/L (or more conservatively, above 4.5 mIU/L) are considered elevated 2
• The geometric mean TSH in disease-free populations is 1.4 mIU/L 3

Interpreting Abnormal TSH Results

High TSH (>4.5 mIU/L)

Always confirm with repeat testing after 3-6 weeks before initiating treatment, as 30-60% of elevated TSH values normalize spontaneously. 2, 3

When confirming, measure both TSH and free T4 to distinguish:

• Subclinical hypothyroidism: Elevated TSH with normal free T4 2
• Overt hypothyroidism: Elevated TSH with low free T4 4

Treatment Algorithm Based on TSH Level:

TSH >10 mIU/L:

• Initiate levothyroxine therapy immediately, regardless of symptoms 3
• This threshold carries ~5% annual risk of progression to overt hypothyroidism 3
• Start with 1.6 mcg/kg/day in patients <70 years without cardiac disease 3
• Start with 25-50 mcg/day in patients >70 years or with cardiac disease, titrating slowly 3

TSH 4.5-10 mIU/L:

• Do NOT routinely treat asymptomatic patients 3
• Monitor TSH every 6-12 months 3
• Consider treatment in specific situations:
  • Symptomatic patients (fatigue, weight gain, cold intolerance) 3
  • Pregnant women or those planning pregnancy 3
  • Positive anti-TPO antibodies (4.3% vs 2.6% annual progression risk) 3
  • Presence of goiter or infertility 3

Low TSH (<0.4 mIU/L)

Measure free T4 and free T3 to distinguish subclinical from overt hyperthyroidism. 4

• Subclinical hyperthyroidism: Low TSH with normal free T4/T3 2
• Overt hyperthyroidism: Low TSH with elevated free T4 or T3 2

Management Based on TSH Level:

TSH <0.1 mIU/L:

• Consider treatment, especially if age >60, cardiac disease, or osteoporosis risk 3
• Carries 3-5 fold increased risk of atrial fibrillation 3
• Associated with increased fracture risk in women >65 years 3

TSH 0.1-0.45 mIU/L:

• Monitor every 3-12 months 3
• Treat if symptomatic or high-risk features present 3

Critical Pitfalls to Avoid

Never treat based on a single abnormal TSH value without confirmation testing. 2, 3

Recognize transient causes of TSH elevation:

• Acute illness or hospitalization (TSH can be transiently elevated) 5
• Recovery phase from thyroiditis 3
• Recent iodine exposure (e.g., CT contrast) 3
• Certain medications 2

In patients on levothyroxine with suppressed TSH (<0.1 mIU/L):

• Approximately 25% of patients are unintentionally overtreated 3
• Reduce dose by 25-50 mcg immediately to prevent atrial fibrillation, osteoporosis, and cardiac complications 3

Before initiating levothyroxine in suspected central hypothyroidism:

• Always rule out adrenal insufficiency first 3
• Starting thyroid hormone before corticosteroids can precipitate life-threatening adrenal crisis 3

Monitoring During Treatment

Recheck TSH and free T4 every 6-8 weeks after any dose adjustment until stable. 3, 4

• Target TSH range: 0.5-4.5 mIU/L for primary hypothyroidism 3
• Once stable, monitor every 6-12 months 3, 4
• Free T4 helps interpret ongoing abnormal TSH during therapy, as TSH may take longer to normalize 3, 4

Special Populations

Elderly patients (>70 years):

• TSH reference range shifts upward (upper limit ~5.9 mIU/L for ages 70-79) 1
• Start levothyroxine at lower doses (25-50 mcg/day) 3
• Titrate more slowly to avoid cardiac complications 3

Pregnant women or planning pregnancy:

• Treat any TSH elevation immediately 3
• Target TSH <2.5 mIU/L in first trimester 3
• Levothyroxine requirements increase 25-50% during pregnancy 3