Introduction: Abnormal thyroid function is the most common disease in endocrinology and its screening of made by plasmatic (TSH), followed by (fT4) determination if abnormal. (fT3) determination is only used when pathological (TSH) and normal (fT4) are found, even if there is no evidence supporting this procedure, apart from the greater technical difficulty of measuring (fT3) due to its lower plasma concentration.
Material and Methods: Observational retrospective study of every sample containing TSH determination in a central laboratory of a tertiary referral center in a non-iodine deficient area from 02/01/2017 to 03/31/2017. In every sample with (TSH)<0.1 mUI/l, (fT4) and (fT3) were measured. In patients without thyroxine treatment, their diagnosis was identified by their electronic medical records.
Results: We found 389 samples with (TSH)<0.1, 5 of them belonged to duplicated patients and 34 of them were not properly registered on the clinical record. Out of the remaining 350 samples, 168 were being treated with thyroxine, 64 with antithyroid drugs and 118 were not being treated. Out of the 182 non hypothyroid patients, 103(61.3%) showed normal (fT4) and (fT3), 41(22.5%) showed both hormones elevated, 32(17.6%) only elevated (fT3) and 6(3.3%) only elevated (T4L). Out of those 6 patients, 3 were amiodarone related thyroiditis, 2 had an excessive iodine supplementation and 1 was toxic multinodular goiter. 68 patients had TSI mediated Graves disease, 24(35.3%) of them had normal (fT4) and (fT3), 26(38.2%) of them had both hormones elevated and 18(26.5%) of them only elevated (fT3). The mean relative level of (fT3) according to upper limit of normal (xULN) among the non-hypothyroid patients was 1.19xULN (S.D.:0.81) and the mean (fT4) level was 0.93×ULN (S.D.:0.46). Among the non-hypothyroid patients with both hormones above the ULN the mean level of (fT3) was 2.25xULN (S.D.:1.11) and the mean level of (fT4) was 1.54xULN (S.D.:0.58). Comparing both results with the Wilcoxon test, there was statistically significant differences in both of them (P<.001).
Discussion: The (fT3) level is significantly higher than (fT4) level among hyperthyroid non levothyroxine dependent patients. (fT3) determination following a supressed (TSH) determination allowed detecting 73 out of 79 patients with elevated thyroid hormones, while the (fT4) determination allowed detecting 47 out of 79 patients. Measuring fT3 when a suppressed TSH may be more cost effective than the usual practice of determining fT4.
18 - 21 May 2019
European Society of Endocrinology