[Discordance of total plasma l-triiodothyronine and l-thyroxine in thyroid diseases]

Radioimmunoassay (RIA) for plasma T3 has been proposed in the diagnosis of thyroid disease. 1 171 simultaneous determination (RIA) of plasma Thyrotropin-Stimulating Hormone (TSH), triiodothyronine (T3) and thyroxine (T4) have been performed in unselected patients under suspicion of a thyroid disease. 81.8% of the cases showed a good agreement between T3 and T4 levels. The remaining 18.2% can be divided into 6 groups: 1. High T3-normal T4 : 51% of the discrepancies: Graves disease untreated (16%) or treated (4%); sporadic goiter (13%); growth failure (8%); hypothyroidism under treatment (6%); thyroiditis, Graves ophtalmopathy, toxic adenoma (4%); 2. High T3-low T4 : 6.5%; sporadic goiter (3%); hypothyroidism (3%); 3. Normal T3-low T4 : 22%: Graves disease under treatment (6%); sporadic goiter (4.5%); subacute thyroiditis (3%); hypothyroidism (3%); miscellaneous (5.5%); 4. Normal T3-High T4 : 9.5%; thyroidectomized patients for cancer well-substituted with T4 (6%); miscellaneous (3.5%) including 1 case of Graves disease; 5. Low T3-normal T4 : 8% 8%: sporadic goiter (4%); hypothyroidism (2%); treated hyperthyroidism (2%); 6. Low T3-high T4 : 3% : including 1 case of hyperthyroidism and 2 cases of thyroidectomized patients well-substituted with T4. In conclusion discrepancies between T3 and T4 levels have been observed mainly in untreated or treated Graves disease (high T3-normal or low T4) and in hypothyroidism (normal or high T3-low T4). The basal TSH and TSH peak after I.V. injection of TRH (65 cases) are in better agreement with T4 level than with T3 level. Indeed, both TRS values are high when T4 is low while T3 is normal of high.