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

Abstract

ObjectiveAlthough 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.MethodsA 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 receivedl-T4therapy at the Kobe University Hospital between 2003 and 2013. They were stratified according to serum FT4level (≥1.10 or <1.10 ng/dl), and body temperature (BT), serum free triiodothyronine (FT3) levels, FT3/FT4ratio, and lipid profiles were compared. The effect of GH replacement therapy on thyroid function was also analyzed.ResultsFT3levels and FT3/FT4ratios 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, FT3levels were higher in those with GH replacement therapy (P=0.018).ConclusionIn CeH, patients with median-lower normal levels of serum FT4exhibited lower serum FT3levels and lower BT. These results support the target levels of serum FT4as middle-upper normal levels duringl-T4replacement therapy in patients with CeH.