Overview
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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