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Thyroid Function
Thyroid Function

Free Thyroxine (T4)



  • 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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