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Thyroid Function

Free Thyroxine (T4)

April 28th, 2020


Free T4 is a measure of the thyroxine not bound to serum proteins such as thyroid-binding globulin, albumin or transthyretin.
    • Normal Range

    • 0.8 - 1.8 ng/dL

Elevated Free T4

Elevated T4 with suppressed TSH is suggestive of primary hyperthyroidism, while a normal or elevated TSH may be present with certain conditions or due to false elevation.
  • Causes of Elevated Free T4

  • Reduced TSH

  • Primary hyperthyroidism
  • Normal / Elevated TSH

  • TSH secreting pituitary tumour
  • Thyroid hormone resistance
  • Acute psychiatric illness
  • Amiodarone
  • False Elevation

  • Reduced TBG - androgens, glucocorticoids
  • Familial dysalbuminaemic hyperthyroxinaemia
  • Heterophile antibodies
  • Heparin

Reduced Free T4

Reduced T4 with elevated TSH is suggestive of primary hypothyroidism, which normal or reduced TSH may be present with certain conditions or due to false reduction.
  • Causes of Reduced Free T4

  • Elevated TSH

  • Primary hypothyroidism
  • Reduced / Normal TSH

  • Nonthyroidal illness
  • Recent treatment of hyperthyroidism
  • Central hypothyroidism
  • Congenital TSH or TRH deficiency
  • False Reduction

  • Increased TBG - pregnancy, oral oestrogen, SERMs, hepatitis
  • Drugs - aspirin, phenytoin, carbamazepine
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