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investigations in hypothyroidism

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  • primary hypothyroidism is indicated by an increase in serum TSH concentration above the upper limit of the reference range combined with free T4 levels below the reference range (1,2)
    • in overt hypothyroidism - serum TSH above 10 mU/L + free T4 below the reference range
    • in subclinical hypothyroidism - serum TSH above the reference range + free T4 within the reference range, the test should be repeated after 3-6 months to exclude transient causes of raised TSH (2)
  • free or total serum T3 is generally unhelpful since it may be only slightly reduced in severe hypothyroidism, or may be elevated by illness unrelated to thyroid function, or as a consequence of incorrect drug therapy
  • secondary hypothyroidism is suggested by reduced free or total T4 and TSH below or within normal range; other tests for hypothalamic / pituitary function are necessary
  • other tests of thyroid function - e.g. thyroidal uptake tests, serum cholesterol have limited value in the diagnosis
  • primary myxoedema may demonstrate a normal or exaggerated response of TSH to TRH; a negative TSH response excludes primary hypothyroidism but not a secondary cause
  • antithyroid antibodies are suggestive of aetiology only, screening for antithyroid antibody is also useful in
    • predicting subsequent hypothyroidism in subclinical hypothyroid patients, in pregnant women and post partum women
    • for differential diagnosis of diffuse goitre (3)

Other possible abnormal laboratory tests include:

  • normochromic macrocytic anaemia
  • low sodium
  • increased prolactin
  • CK, AST and LDH may be elevated (due to abnormal muscle membranes) (3)