Median-lower normal levels of serum thyroxine are associated with low triiodothyronine levels and body temperature in patients with central hypothyroidism.

Objective: Although it has been recommended that serum free thyroxine (FT₄) levels should be targeted to middle-upper normal levels during levothyroxine (L-T₄ ) replacement therapy in patients with central hypothyroidism (CeH), the rationale has not been clarified. Methods: A retrospective single-center study enrolled 116 patients with hypothyroidism (CeH, n = 32; total thyroidectomy (Tx), n = 22; primary hypothyroidism (PH), n = 33; and control benign thyroid nodule (C), n = 29). The patients had received L-T₄ therapy at the Kobe University Hospital between 2003 and 2013. They were stratified according to serum FT₄ level (>1.10 or <1.10 ng/dl), and body temperature (BT), serum free triiodothyronine (FT₃) levels, FT₃/FT₄ ratio, and lipid profiles were compared. The effect of GH replacement therapy on thyroid function was also analyzed. Results: FT₃ levels and FT₃/FT₄ ratios were significantly lower in patients with CeH than in patients with PH (P<0.05) or C (P<0.05). In patients with FT₄ <1.10 ng/dl, BT was significantly lower in patients with CeH (P=0.002) and Tx (P=0.005) than in patients with PH, whereas no differences were found in patients with FT₄ >1.10 ng/dl. In patients with CeH, FT₃ levels were higher in those with GH replacement therapy (P=0.018). Conclusion: In CeH, patients with median-lower normal levels of serum FT₄ exhibited lower serum FT₃ levels and lower BT. These results support the target levels of serum FT₄ as middle-upper normal levels during L-T₄ replacement therapy in patients with CeH. [ABSTRACT FROM AUTHOR]