The T4-free thyroxine index was used as the thyroid function test of first choice in a district general hospital for one year. Ninety-two patients were assessed both initially and at follow up by a single physician. The index agreed with clinical assessment in 69 out of 74 patients in whom diagnosis was unequivocal. There was initial doubt about thyroid status in 14 patients; after re-assessment the index agreed with status in seven cases, in three cases there is still some uncertainty, whilst the index was at variance with clinical status in four patients. Some possible causes of discrepancy between clinical thyroid status and the index are a low index in euthyroid patients due to a fall in serum thyroxine-binding prealbumin (`sick euthyroid'), a raised index in euthyroid patients due to latent thyroid heart disease, and a normal index in thyrotoxicosis due to preferential secretion of triiodothyronine.
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