Samuels Mary H, Kolobova Irina, Antosik Megan, Niederhausen Meike, Purnell Jonathan Q, Schuff Kathryn G
Division of Endocrinology, Diabetes and Clinical Nutrition, Oregon Health & Science University, Portland, Oregon 97239.
Penn State Health St. Joseph Family Medicine Residency, Reading, Pennsylvania 19603.
J Clin Endocrinol Metab. 2017 Jul 1;102(7):2533-2542. doi: 10.1210/jc.2017-00224.
It is not clear whether upper limits of the thyrotropin (TSH) reference range should be lowered. This debate can be better informed by investigation of whether variations in thyroid function within the reference range have clinical effects. Thyroid hormone plays a critical role in determining energy expenditure, body mass, and body composition, and therefore clinically relevant variations in these parameters may occur across the normal range of thyroid function.
This was a cross-sectional study of 140 otherwise healthy hypothyroid subjects receiving chronic replacement therapy with levothyroxine (L-T4) who had TSH levels across the full span of the laboratory reference range (0.34 to 5.6 mU/L). Subjects underwent detailed tests of energy expenditure (total and resting energy expenditure, thermic effect of food, physical activity energy expenditure), substrate oxidation, diet intake, and body composition.
Subjects with low-normal (≤2.5 mU/L) and high-normal (>2.5 mU/L) TSH levels did not differ in any of the outcome measures. However, across the entire group, serum free triiodothyronine (fT3) levels were directly correlated with resting energy expenditure, body mass index (BMI), body fat mass, and visceral fat mass, with clinically relevant variations in these outcomes.
Variations in thyroid function within the laboratory reference range have clinically relevant correlations with resting energy expenditure, BMI, and body composition in L-T4-treated subjects. However, salutary effects of higher fT3 levels on energy expenditure may be counteracted by deleterious effects on body weight and composition. Further studies are needed before these outcomes should be used as a basis for altering L-T4 doses in L-T4-treated subjects.
促甲状腺激素（TSH）参考范围的上限是否应降低尚不清楚。通过调查参考范围内甲状腺功能的变化是否具有临床影响，可以更好地为这场辩论提供信息。甲状腺激素在决定能量消耗、体重和身体组成方面起着关键作用，因此在甲状腺功能的正常范围内，这些参数可能会出现临床相关的变化。
这是一项横断面研究，研究对象为140名接受左甲状腺素（L-T4）长期替代治疗的健康甲状腺功能减退患者，其TSH水平涵盖了实验室参考范围的全跨度（0.34至5.6 mU/L）。受试者接受了能量消耗（总能量消耗和静息能量消耗、食物的热效应、身体活动能量消耗）、底物氧化、饮食摄入和身体组成的详细测试。
TSH水平处于正常低限（≤2.5 mU/L）和正常高限（>2.5 mU/L）的受试者在任何一项结局指标上均无差异。然而，在整个研究组中，血清游离三碘甲状腺原氨酸（fT3）水平与静息能量消耗、体重指数（BMI）、体脂量和内脏脂肪量直接相关，这些结局存在临床相关的变化。
在实验室参考范围内，甲状腺功能的变化与接受L-T4治疗的受试者的静息能量消耗、BMI和身体组成存在临床相关的关联。然而，较高的fT3水平对能量消耗的有益作用可能会被对体重和身体组成的有害作用所抵消。在将这些结局用作改变L-T4治疗患者L-T4剂量的依据之前，还需要进一步的研究。
