What are the normal lab values for Thyroid Stimulating Hormone (TSH), free Triiodothyronine (T3), and free Thyroxine (T4) and how do you interpret them to diagnose hyperthyroidism and hypothyroidism.

Normal Thyroid Lab Values and Their Interpretation in Thyroid Disorders

Normal thyroid lab values are TSH 0.45-4.5 mIU/L, free T4 within reference range, and free T3 within reference range. In hyperthyroidism, TSH is suppressed below 0.45 mIU/L while free T4 and/or free T3 are elevated or normal. In hypothyroidism, TSH is elevated above 4.5 mIU/L while free T4 is low or normal. 1

Normal Reference Ranges

• TSH (Thyroid Stimulating Hormone): 0.45-4.5 mIU/L 1

  • Based on NHANES III data from a disease-free population 1
  • Some variation exists with age (slightly broader range in elderly: 0.4-5.9 mIU/L for 70-79 years) 2

• Free T4 (Thyroxine): Laboratory-specific reference range 1, 3

  • More accurate than total T4 as it's not affected by binding protein variations 3

• Free T3 (Triiodothyronine): Laboratory-specific reference range 1, 3

  • More accurate than total T3 as it's not affected by binding protein variations 3

Lab Patterns in Hyperthyroidism

• TSH: Suppressed below the lower limit of normal (<0.45 mIU/L) 1

  • Often undetectable (<0.01 mIU/L) in overt hyperthyroidism 1
  • In subclinical hyperthyroidism, TSH is low while free T4 and T3 remain normal 1

• Free T4: Normal or elevated 1, 3

  • May be within normal range in T3-toxicosis 3, 4

• Free T3: Normal or elevated 1, 3

  • Often disproportionately elevated compared to free T4 in Graves' disease 3, 4
  • Most sensitive marker for hyperthyroidism diagnosis 4

Lab Patterns in Hypothyroidism

• TSH: Elevated above the upper limit of normal (>4.5 mIU/L) 1

  • In subclinical hypothyroidism, TSH is elevated while free T4 remains normal 1
  • Approximately 75% of patients with elevated TSH have values <10 mIU/L 1

• Free T4: Normal or low 1, 5

  • Low in overt hypothyroidism 5
  • Normal in subclinical hypothyroidism 1

• Free T3: Normal or low 1, 5

  • May remain normal in mild or subclinical hypothyroidism 3
  • Less sensitive than free T4 for diagnosing hypothyroidism 3, 5

Diagnostic Algorithm

1. Initial screening: Measure TSH first 5

   • If normal (0.45-4.5 mIU/L): Likely euthyroid with >99% accuracy 2
   • If abnormal: Proceed to free T4 measurement 5, 2

2. If TSH is elevated (>4.5 mIU/L):

   • With low free T4: Overt primary hypothyroidism 1, 5
   • With normal free T4: Subclinical hypothyroidism 1

3. If TSH is suppressed (<0.45 mIU/L):

   • With elevated free T4 and/or free T3: Overt hyperthyroidism 1
   • With normal free T4 and free T3: Subclinical hyperthyroidism 1
   • With normal free T4 but elevated free T3: T3-toxicosis 3, 4

Common Pitfalls and Caveats

• Non-thyroidal illness: Can cause low TSH, low T3, and normal/low T4 without actual thyroid dysfunction 1

• Medications: Dopamine, glucocorticoids, and dobutamine can suppress TSH without causing hyperthyroidism 1

• Pregnancy: Can cause physiologically low TSH with normal free hormone levels 1

• Central hypothyroidism: Low/normal TSH with low free T4 due to pituitary or hypothalamic dysfunction 1, 5

• Heterophilic antibodies: Can cause falsely elevated TSH values 1, 2

• Recovery phase: TSH may transiently increase during recovery from severe illness or thyroiditis 1

• Binding protein abnormalities: Can affect total T4/T3 levels but not free hormone levels, highlighting the importance of measuring free hormones 3