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Int J Thyroidol. 2019 May;12(1):28-34. English. Original Article. https://doi.org/10.11106/ijt.2019.12.1.28
Jeon MJ , Lee SH , Lee JJ , Han MK , Kim HK , Kim WG , Kim TY , Kim WB , Shong YK , Ryu JS .
Department of Internal Medicine, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea.
Department of Nuclear Medicine, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea. jsryu2@amc.seoul.kr
Division of Nuclear Medicine, Department of Radiology, Hallym University Kangnam Sacred Heart Hospital, Hallym University College of Medicine, Seoul, Korea.
Departments of Clinical Epidemiology, Biostatistics, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea.
Health Promotion Center, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea.
Abstract

Background and Objectives

Levothyroxine (L-T4) monotherapy to normalize TSH level might be not sufficient to restore serum free triiodothyronine (fT3) levels in hypothyroid patients. This study aimed to compare the thyroid hormone levels in euthyroid L-T4 treated athyreotic patients and euthyroid healthy control subjects.

Materials and Methods

We included 69 euthyroid L-T4 treated athyreotic female patients after total thyroidectomy and radioactive iodine ablation therapy and 90 euthyroid healthy female. Serum fT3 and free thyroxine (fT4) levels were simultaneously measured using two different assay kits (A and B).

Results

The serum fT4 level was higher in the athyreotic patients (kit A: p<0.001, kit B: p=0.046), and the serum fT3 level was higher in control subjects (kit A: p=0.047, kit B: p=0.102). In the control group, the serum fT3 level was stable and not correlated with the TSH level (kit A: tau=−0.10, p=0.18, kit B: tau=−0.06, p=0.40). However, in the patient group, the serum fT3 level was negatively correlated with the TSH level (kit A: tau=−0.22, p=0.012, kit B: tau=−0.31, p<0.001). All thyroid hormone parameters measured by kit A showed higher area under the curve values than those measured by kit B for distinguishing the patients from the control subjects.

Conclusion

The serum fT3 levels in L-T4 treated athyreotic euthyroid patients were significantly lower than and varied from those of euthyroid healthy control subjects. Thus, L-T4 monotherapy might not be appropriate for some athyreotic patients to maintain optimal T3 levels.

Copyright © 2019. Korean Association of Medical Journal Editors.