- 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)
NICE suggest tests for people with confirmed primary hypothyroidism (4)
Adults
Consider measuring thyroid peroxidase antibodies (TPOAbs) for adults with TSH levels above the reference range, but do not repeat TPOAbs testing.
Children and young people
Measure TPOAbs for children and young people with TSH levels above the reference range, with possible repeat TPOAbs testing at the time of transition to adult services.
NICE state:
- ask adults, children and young people with suspected thyroid dysfunction about their biotin intake because a high consumption of biotin from dietary supplements may lead to falsely high or low test results
Reference: