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Investigations of Hypothyroidism

Case scenario
A 64 year old woman consulted her medical practitioner about her general lassitude

Clinical history
•    Included weight gain
•    Episodes of constipation
•    A blood specimen was taken for thyroid function tests.

Serum Result Unit Ref Range
tT4 52 nmol/L 60-160
T3 1.3 (Normal) nmol/L 1.2-2.8
TSH 28 mIU/L 0.2-5.6



  • Both the tT4 and fT4 are less than the reference range, but since the patient’s own reference interval is unknown; a biochemical diagnosis cannot be made from these results alone.
  • Serum TSH secretion provides a direct indicator in the individual patient of the bioavailability of circulating thyroid hormone
  • The TSH concentration in the above case in markedly raised
  • The high TSH is therefore diagnostic of primary hypothyroidism
  • TSH value greater than twice the upper limit of the reference interval is a biochemical definition of primary hypothyroidism.

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