Serum Free T3 to Free T4 Ratio as a Useful Indicator for Differentiating Destruction Induced Thyrotoxicosis from Graves' Disease

Abstract

Introduction: Thyrotoxicosis is a common disease encountered in Endocrine Outpatient Department (OPD). Two common causes of thyrotoxicosis are Destruction Induced Thyrotoxicosis (DIT) and Graves’ Disease (GD). Differentiating DIT and GD based on clinical findings, is often not possible due to nonspecific symptoms. Thyroid scan is considered most reliable method for differentiating DIT and GD. Aim: To differentiate DIT and GD using the ratio of free triiodothyronine (fT3 ) and free thyroxine (fT4 ) thus avoiding thyroid technetium scan which is expensive and not accessible in developing countries like Nepal. Materials and Methods: Patients attending Endocrine OPD with diagnosis of thyrotoxicosis at their first visit in which Thyroid technetium scan could be done were taken as sample. The study was conducted from mid-June 2016 to February 2017 and total 55 samples were taken. Only selected cases were taken where diagnostic dilemma was present. Report of Thyroid Function Test (TFT) of patient at their first visit and findings of thyroid scan were recorded. Ratio of freeT3 and freeT4 was obtained in each case. ROC curve was plotted and the cut off value for differentiation of DIT and GD was obtained. All data were analysed using SPSS software version 20.0. Results: Mean ratio of fT3 to fT4 in GD and DIT was 0.395 and 0.287 respectively which was significant. On ROC analysis, cut off ratio for differentiating GD and DIT was 0.30 with sensitivity of 87% and specificity of 62.5%. Conclusion: It is recommended that thyroid scan can be avoided if ratio of fT3 and fT4 is less than 0.3 and a diagnosis of DIT can be made.

Keywords

• ft3 (triiodothyronine)
• ft4 (thyroxine)
• thyroid scan