邢萬佳,姜兆順
促甲狀腺素與心血管病危險因素相關(guān)性研究進(jìn)展
邢萬佳,姜兆順*
[摘要]隨著對亞臨床甲狀腺功能減退的認(rèn)識及促甲狀腺素受體在甲狀腺外分布的發(fā)現(xiàn),研究者們開始關(guān)注促甲狀腺素的甲狀腺外效應(yīng)及其水平改變的臨床意義。筆者回顧性分析了近年關(guān)于亞臨床甲狀腺功能減退及促甲狀腺素水平與血壓、血脂譜、體重指數(shù)、代謝綜合征等心血管疾病危險因素相關(guān)性的研究,結(jié)果提示促甲狀腺素在正常范圍內(nèi)的高水平時,與高血壓、脂代謝異常、肥胖、代謝綜合征乃至因心血管死亡仍可能存在相關(guān)性。
[關(guān)鍵詞]促甲狀腺激素;血脂;血壓;亞臨床甲狀腺機能減低;心血管病
心血管系統(tǒng)是甲狀腺激素 (thyroid hormone,TH)的主要靶器官之一,因此甲狀腺功能異常與心血管病變(cardiovascular disease,CVD)密切相關(guān)[1]。臨床上甲狀腺功能減退被視為CVD的重要危險因素之一[2],關(guān)于其與動脈粥樣硬化之間的相關(guān)性已得到共識[3],其主要機制在于TH水平異常。關(guān)于以TH水平正常、TSH異常升高為特征的亞臨床甲狀腺功能減退(subclinical hypothyroidism,SCH)與CVD之間是否存在相關(guān)性目前尚存在爭議[4-7]。有研究顯示,大多數(shù)CVD發(fā)生于甲狀腺功能正常個體,在血管病變的患者中,正常較高的TSH水平與心肌梗死危險性增加相關(guān)[8]。在肥大型心肌病患者中的研究亦提示[9],TSH在正常高水平的患者左心室舒張功能障礙更為嚴(yán)重。但也有研究結(jié)論與之相左[10,11],甚至有研究提示在CVD患者中,高的TSH水平與降低的全因及心血管死亡率相關(guān)[12]。
TSH的傳統(tǒng)功能被認(rèn)為是與表達(dá)于甲狀腺濾泡細(xì)胞膜上的特異性受體TSHR結(jié)合而實現(xiàn)的。自發(fā)現(xiàn)TSHR mRNA在人眼球后組織中存在表達(dá)[13]以來,諸多研究提示,TSHR可廣泛表達(dá)于甲狀腺外組織中,包括心肌細(xì)胞[14]與脂肪細(xì)胞[15]。 不僅在甲狀腺外表達(dá)廣泛,而且相當(dāng)多的研究顯示,在不同的甲狀腺外組織中存在TSH-TSHR相互作用。例如,TSHR在肝細(xì)胞具備功能性表達(dá)[16],且參與了脂肪細(xì)胞的生理調(diào)控[17]。迄今為止,關(guān)于TSH的甲狀腺外作用尚存在爭議,筆者主要就TSH水平與CVD危險因素的相關(guān)性研究進(jìn)展進(jìn)行綜述回顧。
甲狀腺功能亢進(jìn)與甲狀腺功能減退均與高血壓相關(guān),但SCH是否與升高的血壓(BP)相關(guān)并無定論[18,19]。一項基于社區(qū)人群的研究[20]顯示,與甲狀腺功能正常者相比,SCH患者的收縮壓(SBP)與舒張壓(DBP)并無顯著差異。部分研究[19,21,22]得到相似結(jié)果。但也有研究報道SCH患者SBP與DBP均較甲狀腺功能正常者顯著升高,校正了其他CVD危險因素后,僅SBP與SCH顯著相關(guān)[23]。因此,SCH可能與高血壓風(fēng)險相關(guān)[18],針對SCH適當(dāng)治療將TSH水平降低后或可改善BP,從而對CVD的預(yù)防與治療發(fā)揮遠(yuǎn)期效應(yīng)。
目前尚不知TSH在正常范圍內(nèi)的細(xì)微變化是否對BP產(chǎn)生影響。在甲狀腺功能正常的女性中[24],與TSH位于下四分位數(shù)(0.5~1.31 mIU/L)者相比,TSH水平在上四分位數(shù)(2.61~4.49 mIU/L)者SBP顯著升高,TSH每增加 1 mIU/L,相應(yīng) SBP增加1.53 mmHg。后續(xù)研究亦支持上述結(jié)果[25,26]。在兒童與青春期少年中的研究也提示,高的血漿TSH水平與SBP及DBP呈正相關(guān)[18]。與之相似,在原發(fā)性高血壓患者中,有報道TSH水平與SBP及DBP呈線性正相關(guān)[27]。此外,學(xué)齡兒童中,在正常范圍內(nèi)TSH水平的增加與BP增高相關(guān),在男孩中尤為明顯[28]。最新的一項研究亦提示TSH水平與高血壓呈正相關(guān)關(guān)系[29]。
TSH與BP相關(guān)的確切機制不明。有研究者發(fā)現(xiàn)TSH水平在正常上限(2.1~4.0 mIU/L)的女性血管反應(yīng)性出現(xiàn)異常[30],提示TSH或可通過損害血管功能對血壓產(chǎn)生影響。此外,應(yīng)考慮到遺傳因素對BP及TSH水平的影響。一項針對高血壓家族的研究[31]提示,家族成員聚集性呈現(xiàn)正常高水平的TSH (2.0~5.0 mIU/L)。與對照相比,原發(fā)性高血壓動物模型——自發(fā)性高血壓大鼠亦表現(xiàn)為TSH水平升高[32]。近期研究[33]提示,血清TSH水平與動脈壓及高血壓呈橫斷面正相關(guān),其中與高血壓相關(guān)的OR 為1.76(95%CI=1.24~2.50,P=0.002),但TSH與為期5年的血壓變化之間并無相關(guān)性。因此,TSH與血壓之間的關(guān)系有待進(jìn)一步研究闡明。
與甲狀腺功能正常者相比,SCH患者血脂譜是否變化存在爭議[5,34,35]。部分研究提示,SCH患者三酰甘油(TG)顯著升高[36],而高密度脂蛋白膽固醇(HDL-C)顯著降低[37]。最新研究提示,SCH中TSH水平與脂蛋白(a)、低密度脂蛋白膽固醇(LDL-C)及總膽固醇(TC)呈顯著正相關(guān)[38]。
關(guān)于在正常參考值范圍內(nèi)TSH水平與血脂譜之間相關(guān)性的研究尚無定論。將TSH正常參考值范圍定義為0.3~4.8 mIU/L時,研究發(fā)現(xiàn),在不同TSH水平區(qū)間 TG及 HDL-C水平均無顯著差異[39]。Westerink等[40]也報道了相似結(jié)果。然而,在Hunt研究[41]中,以0.5~3.5 mIU/L作為正常TSH參考范圍,結(jié)果提示,隨著TSH水平在正常范圍內(nèi)升高,TC、LDL-C及非HDL-C均與TSH呈顯著線性正相關(guān),而HDL-C與TSH呈線性負(fù)相關(guān)。另一項針對印度女性的研究發(fā)現(xiàn),TSH位于正常參考值范圍上限時 (TSH>2.1 μU/ml),其水平與較高的TC、TG、LDL-C及較低的HDL-C水平相關(guān)[29]。來自意大利的一項納入490例無甲狀腺病變且血糖控制基本滿意的2型糖尿病患者的研究發(fā)現(xiàn),將其正常參考值范圍內(nèi)的TSH水平分組后,TSH在正常最高組(1.91~4.00 mIU/L)者,TG明顯高于TSH在正常最低組(0.28~0.90 mIU/L)[(163.8±102.9)mg/dl vs (128.3±65.5)mg/dl,P=0.002)[42]。有學(xué)者推測,即便在正常參考值范圍內(nèi),TSH可以通過上調(diào)肝羥甲基戊二酰輔酶A還原酶促進(jìn)膽固醇合成,導(dǎo)致增高的TSH與不利的血脂譜之間呈現(xiàn)相關(guān)性[43]。因此,TSH可能對心血管健康具有長期的不利影響。同時,應(yīng)當(dāng)注意的是,目前所有關(guān)于TSH水平與血脂譜之間的相關(guān)性研究均呈現(xiàn)較弱或中等相關(guān),其臨床意義有待進(jìn)一步闡明。
超重與肥胖是CVD的重要危險因素。鑒于TSHR可在脂肪細(xì)胞中表達(dá)[17,44,45],有關(guān)TSH在脂肪細(xì)胞分化及體重(BW)方面的作用受到關(guān)注。對比野生型對照及TSHR基因敲除小鼠,發(fā)現(xiàn)兩者脂肪細(xì)胞平均大小存在顯著差異[17],而來自甲狀腺功能正常者的研究[46]并未發(fā)現(xiàn)TSH水平與體重指數(shù)(BMI)之間存在相關(guān)性(P=0.29)。與之相反,另一項基于人群的研究[47]提示,在正常參考值范圍內(nèi)的TSH水平與BMI正相關(guān),經(jīng)校正年齡與吸煙因素后,女性TSH水平每升高1 mIU/L,BMI增加0.41 kg/ m2,男性相應(yīng)BMI增幅為0.48 kg/m2。針對中國人的研究亦顯示,TSH在0.3~4.8mIU/L的正常參考值范圍內(nèi)與BMI呈正相關(guān)關(guān)系 (β coefficient=0.921,P= 0.022)[39]。一項在甲狀腺功能正常女性中的研究[29]亦發(fā)現(xiàn),與TSH低于中位數(shù)者相比,TSH超過中位數(shù)(>2.1 μU/ml)的個體更加肥胖。Giandalia等[42]對TSH在正常范圍內(nèi)的2型糖尿病患者的研究亦提示高水平TSH組患者具有更高的BMI。導(dǎo)致上述相關(guān)性的確切機制尚不清楚,在喂食誘導(dǎo)的肥胖小鼠及具有較高BMI的人群中,脂肪組織中TSHR表達(dá)增加[48]。因此,推測在特定條件下TSHR表達(dá)的調(diào)控異常,可能通過過多的脂肪生成參與了肥胖的發(fā)生。究竟TSHR在脂肪細(xì)胞中的表達(dá)是否增加及其作用是如何發(fā)揮尚有待闡明。
一項為期3.5年前瞻性研究發(fā)現(xiàn),盡管基線TSH水平與BW變化無相關(guān)性,但無論男性還是女性,隨訪中TSH水平的增高與BW增加呈正相關(guān)[49]。后續(xù)一項長達(dá)10.5年、基于人群的隨訪研究也得到了相似結(jié)論[50]。因此,提示TSH在正常范圍內(nèi)的中等程度升高可能與體重增加相關(guān)。但來自德國的臨床觀察[51]發(fā)現(xiàn),盡管肥胖的女孩TSH水平更高,但減重5%后TSH水平降低,故認(rèn)為TSH在肥胖個體中的升高似乎是肥胖的結(jié)果而非導(dǎo)致肥胖的原因。另有學(xué)者推測,TSH水平升高可能是針對BW增加的適應(yīng)性反應(yīng)[52]。此外,其他因素,諸如未診斷的SCH,下丘腦-垂體功能異?;騎SH抵抗均應(yīng)在分析TSH與BW的相關(guān)性時加以考慮[53]。
代謝綜合征(metabolic syndrome,MS)是肥胖、血脂異常、胰島素抵抗、高血糖、高血壓等諸多代謝異常在個體的聚集,同時也是CVD發(fā)生與進(jìn)展的重要危險因素。盡管SCH亦被認(rèn)為是MS發(fā)生的危險因素之一[54,55],TSH水平與MS是否相關(guān)仍存疑。來自甲狀腺功能正常的絕經(jīng)后女性的研究顯示[56],TSH水平與MS危險性相關(guān) (OR=1.55,P<0.001)。Boggio等[29]的研究也觀察到類似結(jié)果。另一項在甲狀腺功能正常的韓國女性中的前瞻性研究[57]顯示,在基線時經(jīng)校正后的TSH水平與血清胰島素及HOMA胰島素抵抗指數(shù)(HOMA-IR)的相關(guān)性并不顯著,但隨訪3年后,顯示經(jīng)校正后的高水平的TSH與胰島素水平變化及HOMA-IR的變化顯著相關(guān)(β分別為0.166及0.164,P均為0.008)。進(jìn)一步的logistic回歸分析提示,MS與增加的TSH水平相關(guān)(OR=1.103,P=0.041),故提示在韓國女性中高水平TSH可能預(yù)示MS的發(fā)生。
盡管部分小規(guī)模臨床研究支持SCH與增加的CVD風(fēng)險相關(guān)[4,58],一項在老年人群的研究卻顯示,亞臨床甲狀腺功能異常與增加的全因死亡或心血管病死亡無相關(guān)性[59]。大規(guī)模的前瞻性隊列研究均顯示,SCH與老年患者的心力衰竭發(fā)生及進(jìn)展相關(guān),但關(guān)于SCH與冠心?。–HD)發(fā)生及死亡的關(guān)系僅見于年齡低于65歲患者[60]。TSH水平與CVD事件是否相關(guān)尚無定論[5,12],部分研究甚至認(rèn)為,高水平的TSH與全因死亡及心血管死亡呈顯著負(fù)相關(guān)關(guān)系(校正后的OR分別為0.47及0.33,P<0.01 及0.05)[12]。研究者認(rèn)為,在高水平TSH個體中相對低的全因及心血管病死亡可能與低代謝水平有關(guān),此種代謝水平降低的作用超過了因血脂異常導(dǎo)致的動脈粥樣硬化危險,而在較低TSH水平組觀察到的相對較低的死亡率可能與甲狀腺功能亢進(jìn)相關(guān)的抗動脈粥樣硬化作用相關(guān)。一項meta分析[61]顯示,CHD事件與CHD死亡率均隨TSH水平增加而升高,TSH水平在4.5~6.9 mIU/L、7.0~9.9 mIU/ L及 10~19.9 mIU/L三組的CHD死亡相對危險(HR)分別為1.09、1.42及1.58。因此,該研究認(rèn)為SCH是與增加的CHD事件及CHD死亡風(fēng)險相關(guān),在TSH水平超過10 mIU/L,上述相關(guān)性更為顯著。
TSH在正常參考值范圍內(nèi)與CVD事件相關(guān)性的研究較少,一項臨床血管病變患者中的前瞻性隊列研究[8]發(fā)現(xiàn),盡管TSH與中風(fēng)或全因死亡風(fēng)險無相關(guān)性,但TSH在正常范圍內(nèi)每升高1單位,心肌梗死的風(fēng)險提高33%。上述相關(guān)性在非內(nèi)臟型肥胖患者中更為顯著。在著名的HUNT研究中[62],同樣提示女性中正常范圍內(nèi)TSH水平與CVD死亡率存在線性正相關(guān)關(guān)系。
鑒于TSHR在甲狀腺外組織的廣泛表達(dá),研究者推測TSH可能參與了大量的機體功能調(diào)控。大量證據(jù)顯示,在正常范圍內(nèi)或輕度升高的TSH水平與CVD危險因素之間存在可能的相關(guān)性,但仍面臨相當(dāng)多的挑戰(zhàn)。例如,由于循環(huán)中TSH與THs間存在相互作用,研究中很難區(qū)分TSH的效應(yīng)或THs波動的作用,從而對臨床結(jié)果的分析造成困擾[63]。此外,由于TSH的正常參考范圍在不同種族、年齡均存在差異[64],臨床難以設(shè)定適合的TSH正常參考范圍,需要更多的深入研究,進(jìn)一步闡明TSH與CVD危險因子之間的相關(guān)性,從而為臨床干預(yù)提供依據(jù)。
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[2015-05-14收稿,2015-06-13修回][本文編輯:王軍紅]
[中圖分類號]R575+.14
[文獻(xiàn)標(biāo)志碼]A
[作者單位]250031山東濟南,濟南軍區(qū)總醫(yī)院內(nèi)分泌科(邢萬佳,姜兆順)
[通訊作者]姜兆順,Email:jzs012@163.com
Current insight into the associations between thyrotropin and cardiovascular risk factors
XING Wanjia,JIANG Zhao-shun.Department of Endocrinology,the General Hospital of Jinan Military Command,Jinan,Shandong 250031,China
[Abstract]With the recoginition of subclinical hypothyroidism which is characterized by abnormal high thyrotropin(TSH)levels and the wide extra-thyroidal expression of thyrotropin receptor,the clinical implications of TSH outside thyroid gland had aroused great attention.Even within the normal range,TSH might perticipate in the regulation of lipid profiles,blood pressure,body weight and other factors associated with cardiovascular diseases,although the detailed mechanisms remain unclear.TSH might be involved in a wide array of extrathyroidal regulatory mechanisms.There was a possible association between normal or slightly elevated TSH levels and CVD risk factors.
[Key words]Thyroid-stimulating hormone;Blood lipid;Blood pressure;Subclinical hypothyroidism;Cardiovascular disease