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