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We consider that biochemical tests of thyroid function are of little, if any, value clinically in patients receiving thyroxine replacement. Most patients are rendered euthyroid by a daily dose of 100 or 150 micrograms of thyroxine. Further adjustments to the dose should be made according to the patient's clinical response. In our laboratory 36% of all thyroid function tests are performed to monitor thyroxine replacement. To stop doing these tests in such patients would cause considerable saving in the costs of reagents in laboratories using commercial kits.
Our findings emphasise the need for laboratories to make their users aware that the reference ranges for serum thyroxine, free thyroxine, and thyroid stimulating hormone concentrations in patients receiving thyroxine replacement are considerably different from the conventional ranges; they should also point out the limitations of these ranges. This is especially important for general practitioners and non-specialists, who generally rely on the biochemical findings more than specialist endocrinologists do in managing these patients. The conclusions in this paper also have major implications for schemes such as the Scottish Automated Follow-Up Registry,2" which relies on results of thyroid function tests in assessing patients treated by radioiodine or thyroid surgery.