Circulating TSH levels measured with an immunochemiluminometric assay in patients taking drugs interfering with biochemical thyroid status.

Serum TSH was measured using a high sensitivity immunochemiluminometric assay (ICMA) in patients receiving anticonvulsant drugs, heparin or non-steroidal anti-inflammatory agents (NSAIs) and the results compared with those from groups of age- and sex-matched controls. The TSH results have also been compared with those of estimates of free thyroid hormone levels using Amerlex analogue method reagents. All patients were clinically euthyroid and TSH concentrations were in the normal, euthyroid range and did not differ significantly in any group studied. In contrast, Amerlex free T4 levels were significantly reduced in the patients treated with phenytoin (P less than 0.001), carbamazepine (P less than 0.01), sodium valproate (P less than 0.01) and heparin (P less than 0.001). Patients treated with NSAI agents showed no significant change in free T4 levels. Amerlex free T3 levels were also significantly reduced in the patients treated with phenytoin (P less than 0.005), carbamazepine (P less than 0.005) and heparin (P less than 0.001) but not in those treated with sodium valproate or NSAIs. These data support the view that measurement of circulating TSH with an assay of sufficient precision in the relevant range provides a reliable way of assessing thyroid status and could be used to exclude hyperthyroidism in patients taking the medications investigated in this study.