What is the normal range for Thyroid-Stimulating Hormone (TSH) levels and how are abnormal levels treated?

Normal TSH Ranges and Treatment of Abnormal Levels

The normal reference range for TSH is generally 0.45-4.5 mIU/L, with treatment recommendations varying based on the degree of TSH elevation and patient characteristics. 1

Normal TSH Ranges

• The 95% confidence interval for TSH reference values is 0.45-4.12 mIU/L in the general population 2
• The reference range becomes slightly broader in older adults, with values of 0.4-5.9 mIU/L for those aged 70-79 years 2
• Some experts suggest a narrower "optimal" range of 0.4-2.5 mIU/L based on clinical outcomes 3
• Age-specific reference ranges should be considered, as the upper limit increases with age 4

Diagnosis of Thyroid Dysfunction

• TSH is the most sensitive test for initial evaluation of suspected hypothyroidism, with >99% accuracy 5
• Always measure both TSH and Free T4 to properly diagnose thyroid dysfunction 6
• An initially elevated TSH should be confirmed with repeat testing after 2-3 months, as 30-60% of high TSH levels normalize on repeat testing 1, 7
• Thyroid peroxidase (TPO) antibodies help identify autoimmune etiology and predict higher risk of progression to overt hypothyroidism 1

Treatment of Hypothyroidism

Overt Hypothyroidism (Elevated TSH, Low Free T4)

• Levothyroxine replacement is the standard treatment 6
• Dosing depends on patient characteristics:
  • Young, healthy patients: Full replacement dose of approximately 1.6 mcg/kg/day based on ideal body weight 1, 6
  • Elderly patients (>70 years) or those with cardiovascular disease: Start with a lower dose of 25-50 mcg daily and titrate gradually 1, 6

Subclinical Hypothyroidism (Elevated TSH, Normal Free T4)

• For TSH >10 mIU/L: Initiate levothyroxine therapy regardless of symptoms 1
• For TSH between 4.5-10 mIU/L: Treatment decisions should be individualized based on:
  • Age: Treatment may be beneficial for younger patients (<65-70 years) but potentially harmful in elderly patients 7, 4
  • Symptoms: Consider a trial of therapy for symptomatic patients 1, 4
  • Presence of TPO antibodies: Higher risk of progression to overt hypothyroidism 1
  • Pregnancy planning: More aggressive normalization of TSH is warranted 1, 8

Monitoring and Dose Adjustment

• Monitor TSH every 6-8 weeks while titrating hormone replacement 1, 6
• Once stable, check TSH every 6-12 months 1, 6
• Target TSH for primary hypothyroidism: 0.4-2.5 mIU/L (lower half of the reference range) 1, 4, 2
• For pregnant patients: Maintain TSH in trimester-specific reference range 8
• For thyroid cancer patients: TSH targets depend on risk stratification, potentially as low as <0.1 mIU/L 1

Treatment of Hyperthyroidism

• For overt hyperthyroidism (suppressed TSH, elevated Free T4): Options include beta-blockers for symptom control, anti-thyroid medications, radioactive iodine, or surgery 6
• For subclinical hyperthyroidism (suppressed TSH, normal Free T4): Consider treatment for TSH <0.1 mIU/L, especially in elderly patients or those with cardiac disease 6

Common Pitfalls and Considerations

• Overtreatment with levothyroxine can lead to iatrogenic hyperthyroidism in 14-21% of treated patients 1, 6
• Prolonged TSH suppression increases risk for atrial fibrillation, osteoporosis, and cardiovascular mortality 1
• Failure to recognize transient hypothyroidism may lead to unnecessary lifelong treatment 1
• In patients with both adrenal insufficiency and hypothyroidism, steroids should always be started prior to thyroid hormone to avoid an adrenal crisis 1
• For patients with persistent symptoms despite normalized TSH, consider alternative causes or combined T4/T3 therapy, especially in those with type 2 deiodinase polymorphisms 7