Guillaume J, Schussler G C, Goldman J
J Clin Endocrinol Metab. 1985 Apr;60(4):678-84. doi: 10.1210/jcem-60-4-678.
The distribution of thyroid hormones between free solution and their several protein-binding sites during pregnancy was studied under physiological conditions of temperature and pH. Single serum specimens were obtained from individual women at different stages of pregnancy. During the first 5 weeks of pregnancy, mean serum free T4 and free T3 concentrations were 50% higher than in nonpregnant women or women during the third trimester. Free T4 was increased significantly throughout the first trimester, but because of wide variance, free T3 was significantly above control values only during the first 5 weeks. Free T4 and free T3 concentrations decreased to control levels in the third trimester. These changes in free T4 concentrations are consistent with a weak thyrotropic action of hCG, which attained maximal concentrations early in the first trimester and then decreased markedly in the second and third trimesters. TRH testing of women scheduled for abortion in the first and second trimesters revealed marked inhibition of TSH response to TRH in those first trimester women who had elevated free T4 concentrations. The percent free T4 did not decrease during the first 5 weeks, but then declined progressively to term as T4-binding globulin (TBG) affinity, defined as the product of the capacity and affinity constant, progressively increased. T4 bound to TBG (T4-TBG) increased from early in the first trimester to term, and then decreased in postterm pregnancy and postpartum. T4 bound to prealbumin (T4-PA) and to albumin (T4-Alb) decreased significantly in the third trimester compared with either control or first trimester concentrations. The concentration of free T3 was positively correlated with T4-PA (r = 0.25) and T4-Alb (r = 0.31), but not with free T4 (r = 0.18) or T4-TBG (r = -0.30) concentrations. These results suggest that 1) only the high concentrations of hCG present in the first trimester of pregnancy have a thyrotropic effect in excess of normal levels of TSH, and this can be sufficient to suppress the TSH response to TRH; 2) hepatic TBG secretion continues to respond to the continuously rising estrogen levels throughout pregnancy; and 3) T4-PA and T4-Alb, but not free T4 or T4-TBG, are possible precursors for the extrathyroidal generation of T3.
在温度和pH值的生理条件下，研究了孕期甲状腺激素在游离溶液与其几种蛋白质结合位点之间的分布情况。从处于不同孕期的个体女性中获取单一血清样本。在怀孕的前5周，血清游离T4和游离T3的平均浓度比未怀孕女性或孕晚期女性高50%。游离T4在整个孕早期显著升高，但由于差异较大，游离T3仅在最初5周显著高于对照值。游离T4和游离T3浓度在孕晚期降至对照水平。游离T4浓度的这些变化与hCG的弱促甲状腺作用一致，hCG在孕早期达到最高浓度，然后在孕中期和孕晚期显著下降。对计划在孕早期和孕中期进行流产的女性进行TRH检测发现，那些游离T4浓度升高的孕早期女性，其TSH对TRH的反应受到明显抑制。游离T4百分比在最初5周没有下降，但随后随着T4结合球蛋白（TBG）亲和力（定义为容量与亲和常数的乘积）逐渐增加而逐渐下降至足月。与TBG结合的T4（T4-TBG）从孕早期开始增加至足月，然后在过期妊娠和产后下降。与对照或孕早期浓度相比，与前白蛋白（T4-PA）和白蛋白（T4-Alb）结合的T4在孕晚期显著下降。游离T3浓度与T4-PA（r = 0.25）和T4-Alb（r = 0.31）呈正相关，但与游离T4（r = 0.18）或T4-TBG（r = -0.30）浓度无关。这些结果表明：1）仅在怀孕前3个月存在的高浓度hCG具有超过正常TSH水平的促甲状腺作用，这足以抑制TSH对TRH的反应；2）肝脏TBG分泌在整个孕期持续对不断升高的雌激素水平做出反应；3）T4-PA和T4-Alb而非游离T4或T4-TBG可能是甲状腺外T3生成的前体。
