Lewis G F, Alessi C A, Imperial J G, Refetoff S
Department of Medicine, University of Chicago, Illinois 60637.
J Clin Endocrinol Metab. 1991 Oct;73(4):843-9. doi: 10.1210/jcem-73-4-843.
Subnormal free T4 index (FT4I) values (less than 80) with inappropriately normal serum TSH concentrations that could not be attributed to illness or drugs were found in 2.5% of ambulating elderly clinic patients. Six such individuals (three men and three women, aged 68.8 +/- 4.8 yr) were selected for their persistent thyroid test abnormalities and were sex and age matched to six subjects (67.7 +/- 4.9 yr) with normal FT4I (greater than 90) and TSH levels. The former also had low serum total T4 (TT4) and rT3 (TrT3) concentrations, but total T3 (TT3) and basal TSH values were normal and did not differ between the groups. Responses of ACTH, LH, FSH, TSH, and PRL to stimulation with CRH, GnRH, and TRH showed no differences between the two groups, indicating that the normal TSH concentration, inappropriate for the low FT4I level, is not due to generalized hypothalamic or pituitary dysfunction. Administration of 3 g iopanoic acid (IOP) daily for 3 days produced significant increases in the TT4 and TrT3 concentrations to the same degree in both groups. Also, in both groups the IOP-induced suppression of T4 to T3 conversion in the pituitary gland provoked similar increases in basal TSH (280 +/- 47% and 288 +/- 33%) and TSH secretion in response to TRH (173 +/- 7% and 156 +/- 13%). These results indicate that the low FT4I is not the consequence of reduced pituitary TSH reserve. In addition, evidence for normal thyroid gland reserve and the secretion of TSH of normal biological activity was obtained by comparing the acute iodothyronine responses to TRH-induced TSH release in both groups. It is concluded that the normal serum TSH concentration, inappropriate for the low FT4I value in some elderly subjects, is due to an apparent resetting of the thyroid hormone feedback regulation threshold of TSH secretion. It may, in turn, be the result of enhanced pituitary conversion of T4 to T3 or increased T4 uptake by the thyrotrophs.
在2.5%的老年门诊患者中发现，游离甲状腺素指数（FT4I）值低于正常范围（低于80），而血清促甲状腺激素（TSH）浓度却正常，且这种情况不能归因于疾病或药物。选择了6例此类患者（3男3女，年龄68.8±4.8岁），他们的甲状腺检查持续异常，并与6例FT4I正常（大于90）且TSH水平正常的受试者（67.7±4.9岁）进行性别和年龄匹配。前者血清总甲状腺素（TT4）和反三碘甲状腺原氨酸（rT3）浓度也较低，但总三碘甲状腺原氨酸（TT3）和基础TSH值正常，两组之间无差异。促肾上腺皮质激素（ACTH）、促黄体生成素（LH）、促卵泡生成素（FSH）、TSH和催乳素（PRL）对促肾上腺皮质激素释放激素（CRH）、促性腺激素释放激素（GnRH）和促甲状腺激素释放激素（TRH）刺激的反应在两组之间无差异，这表明对于低FT4I水平而言正常的TSH浓度并非由于下丘脑或垂体的广泛功能障碍所致。两组患者连续3天每天口服3 g碘番酸（IOP）后，TT4和rT3浓度均显著升高，且升高程度相同。此外，两组中IOP诱导的垂体中T4向T3转化的抑制均引起基础TSH（分别为280±47%和288±33%）和TSH对TRH反应性分泌（分别为173±7%和156±13%）的相似升高。这些结果表明，低FT4I并非垂体TSH储备减少的结果。此外，通过比较两组对TRH诱导的TSH释放的急性碘甲状腺原氨酸反应，获得了甲状腺储备正常以及具有正常生物活性的TSH分泌的证据。得出的结论是，在一些老年受试者中，对于低FT4I值而言正常的血清TSH浓度是由于TSH分泌的甲状腺激素反馈调节阈值明显重置所致。反过来，这可能是垂体中T4向T3转化增强或促甲状腺细胞对T4摄取增加的结果。
