Endocrinology Book

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ThyroxineAka: Free T4, T4

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  1. See Also
    1. Thyroxine Replacement
    2. Hypothyroidism
    3. Hyperthyroidism
  2. Indication
    1. Evaluation of Thyroid Function
    2. Combined with Thyroid Stimulating Hormone (TSH)
      1. Together most accurate assessment of Thyroid status
  3. Normal
    1. Thyroxine (T4): 4-12 ug/dl
    2. Free Thyroxine (Free T4): 0.7 - 1.9 ng/dl
  4. 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
  5. Increased Thyroxine or Free T4
    1. Grave's Disease
    2. Toxic Multinodular Goiter
    3. Toxic adenoma
    4. Iatrogenic/factitious
    5. Transient Hyperthyroidism
      1. Subacute Thyroiditis,
      2. Hashimoto's Thyroiditis
    6. Rarely
      1. HyperThyroid secondary to pituitary disease
      2. Thyroid Cancer
      3. Amiodarone
  6. 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. PAS
        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

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