(Table 1) Circulating levels of TT3 and TT4, and age, length, girth, and estimated body mass in polar bears (Ursus maritimus) from East Greenland 1999-2001

We investigated the multivariate relationships between adipose tissue residue levels of 48 individual organohalogen contaminants (OHCs) and circulating thyroid hormone (TH) levels in polar bears (Ursus maritimus) from East Greenland (1999-2001, n = 62), using projection to latent structure (PLS) regression for four groupings of polar bears; subadults (SubA), adult females with cubs (AdF_N), adult females without cubs (AdF_S) and adult males (AdM). In the resulting significant PLS models for SubA, AdF_N and AdF_S, some OHCs were especially important in explaining variations in circulating TH levels: polybrominated diphenylether (PBDE)-99, PBDE-100, PBDE-153, polychlorinated biphenyl (PCB)-52, PCB-118, cis-nonachlor, trans-nonachlor, trichlorobenzene (TCB) and pentachlorobenzene (QCB), and both negative and positive relationships with THs were found. In addition, the models revealed that DDTs had a positive influence on total 3,5,3'-triiodothyronine (TT3) in AdF_S, and that a group of 17 higher chlorinated ortho-PCBs had a positive influence on total 3,5,3',5'-tetraiodothyronine (thyroxine, TT4) in AdF_N. TH levels in AdM seemed less influenced by OHCs because of non-significant PLS models. TH levels were also influenced by biological factors such as age, sex, body size, lipid content of adipose tissue and sampling date. When controlling for biological variables, the major relationships from the PLS models for SubA, AdF_N and AdF_S were found significant in partial correlations. The most important OHCs that influenced TH levels in the significant PLS models may potentially act through similar mechanisms on the hypothalamic-pituitary-thyroid (HPT) axis, suggesting that both combined effects by dose and response addition and perhaps synergistic potentiation may be a possibility in these polar bears. Statistical associations are not evidence per se of biological cause-effect relationships. Still, the results of the present study indicate that OHCs may affect circulating TH levels in East Greenland polar bears, adding to the "weight of evidence" suggesting that OHCs might interfere with thyroid homeostasis in polar bears.我们针对东格陵兰地区1999-2001年间的62只北极熊（Ursus maritimus），探究了其脂肪组织中48种单一有机卤素污染物（organohalogen contaminants, OHCs）残留水平与循环甲状腺激素（thyroid hormone, TH）水平之间的多元关联。研究针对四类北极熊群体采用偏最小二乘（projection to latent structure, PLS）回归分析：亚成体（SubA）、带幼崽成年雌性（AdF_N）、不带幼崽成年雌性（AdF_S）以及成年雄性（AdM）。在针对亚成体、带幼崽成年雌性以及不带幼崽成年雌性所构建的显著PLS回归模型中，部分有机卤素污染物对解释循环甲状腺激素水平的变异尤为关键：多溴二苯醚（polybrominated diphenylether, PBDE）-99、PBDE-100、PBDE-153，多氯联苯（polychlorinated biphenyl, PCB）-52、PCB-118，顺式九氯、反式九氯、三氯苯（trichlorobenzene, TCB）以及五氯苯（pentachlorobenzene, QCB），且这些污染物与甲状腺激素水平间存在正向及负向关联。此外，模型结果显示，在不带幼崽的成年雌性群体中，滴滴涕类（DDTs）对总三碘甲状腺原氨酸（total 3,5,3'-triiodothyronine, TT3）具有正向影响；而在带幼崽的成年雌性群体中，17种高氯代邻位多氯联苯组群对总甲状腺素（total 3,5,3',5'-tetraiodothyronine, TT4，又称thyroxine）具有正向影响。由于针对成年雄性的PLS回归模型未达到显著水平，其循环甲状腺激素水平受有机卤素污染物的影响相对较弱。循环甲状腺激素水平同时还受年龄、性别、体尺、脂肪组织脂质含量以及采样日期等生物学因素的影响。在控制生物学变量的前提下，针对亚成体、带幼崽成年雌性与不带幼崽成年雌性的PLS模型所揭示的主要关联，在偏相关分析中仍具有统计学显著性。在显著PLS模型中，对甲状腺激素水平产生影响的关键有机卤素污染物，可能通过作用于下丘脑-垂体-甲状腺（hypothalamic-pituitary-thyroid, HPT）轴的相似机制发挥效应，这提示此类北极熊体内可能同时存在剂量与响应相加的联合效应，乃至协同增强效应。统计关联本身并非生物学因果关系的直接证据。尽管如此，本研究结果仍表明，有机卤素污染物可能会影响东格陵兰北极熊的循环甲状腺激素水平，进一步为“有机卤素污染物可能干扰北极熊甲状腺稳态”这一论断增添了证据权重。