Lab

Thyroxine

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Thyroxine, Free T4, Free Thyroxine, T4

  • Indication
  1. Evaluation of Thyroid Function
  2. Combined with Thyroid Stimulating Hormone (TSH)
    1. Together most accurate assessment of Thyroid status
  • Mechanism
  • Measurement
  1. Radioimmunoassay measures bound serum Thyroxine (T4)
  2. Free Thyroxine (Free T4) measures only active T4
    1. Not affected by Thyroid Binding Globulin (TBG)
    2. Free Thyroxine (Free T4) represents 0.03% of total T4
  • Interpretation
  • Normal
  1. Thyroxine (T4): 4-12 ug/dl
  2. Free Thyroxine (Free T4): 0.7 - 1.9 ng/dl
  3. Pregnancy (Free T4)
    1. First trimester: 0.8 to 1.2 ng/dl
    2. Second trimester: 0.6 to 1.0 ng/dl
    3. Third timester: 0.5 to 0.8 ng/dl
    4. Abbassi (2010) Obstet Gynecol 114(6): 1326-31 [PubMed]
  • Causes
  • Increased Thyroxine (or Free T4)
  1. Most common
    1. Grave's Disease
    2. Toxic Multinodular Goiter
    3. Toxic adenoma
    4. Iatrogenic/factitious
    5. Transient Hyperthyroidism
      1. Subacute Thyroiditis,
      2. Hashimoto's Thyroiditis
  2. Uncommon to Rare
    1. Hyperthyroid secondary to pituitary disease
    2. Thyroid Cancer
    3. Amiodarone
  • Causes
  • Decreased Thyroxine or Free T4
  1. Primary Hypothyroidism
    1. Hashimoto's Thyroiditis
    2. Idiopathic Myxedema
    3. Previous treatment of Hyperthyroidism
    4. Subacute Thyroiditis
    5. Prior neck Radiotherapy
    6. Iodine deficiency or excess
    7. Congenital
    8. Medications
      1. Lithium
      2. P-Aminosalicylic Acid
      3. Sulfonamides
      4. Phenylbutazone
      5. Amiodarone
      6. Thiourea
  2. Secondary Hypothyroidism
    1. Pituitary dysfunction
  3. Tertiary Hypothyroidism
    1. Hypothalamic disease
  4. Very rare
    1. Tissue resistance to Thyroid hormone