Trimester- and method-specific reference intervals for thyroid tests in pregnant Chinese women: methodology, euthyroid definition and iodine status can influence the setting of reference intervals.
OBJECTIVE
The importance of diagnosis and treatment of thyroid dysfunction during pregnancy has been widely recognized. We therefore established trimester- and method-specific reference intervals for thyroid testing in pregnant women according to the NACB recommended criteria. Several factors can affect the setting of reference intervals, in particular manufacturer's methodology, euthyroid definition and iodine status.


METHODS
Cross-sectional dataset analysis.


METHODS
Five hundred and five normal pregnant women at different stages of gestation were rigorously selected for setting reference intervals. All were healthy, iodine sufficient, euthyroid and negative for both serum thyroid peroxidase antibody (TPOAb) and thyroglobulin antibody (TgAb).


METHODS
Thyrotrophin (TSH), total and free thyroxine (TT4 and FT4), total and free triiodothyronine (TT3 and FT3) and anti-TPOAb and anti-TgAb were measured using the Bayer ADVIA Centaur system. Iodine content in drinking water, salt and urine was determined by national standard methods. The 2·5th and 97·5th percentiles were calculated as the reference intervals for thyroid hormone levels during each trimester.


RESULTS
All participants had long-term consumption of iodized salt and median urinary iodine of 150-200 μg/l during each three trimester. The reference intervals for the first, second and third trimesters were, respectively, TSH 0·03-4·51, 0·05-4·50 and 0·47-4·54 mIU/l and FT4 11·8-21·0, 10·6-17·6 and 9·2-16·7 pmol/l. The manufacturer's method, euthyroid definition and iodine status may influence TSH and FT4 reference intervals. Alterations in thyroid hormone concentrations during pregnancy differed at different stage of gestation and to those of a nonpregnant state.


CONCLUSIONS
The trimester- and method-based reference intervals for thyroid tests during pregnancy are clinically appropriate. Some variables should be controlled when establishing reference intervals.
Institute of Endocrinology, and Key Laboratory of Hormone and Development of China Ministry of Health, Tianjin Medical University, 22 Qixiangtai Road, Heping District, Tianjin 300070, China. yanyuqin2005@yahoo.com.cn
Yu-Qin
YQ
Yan
Zuo-Liang
ZL
Dong
Ling
L
Dong
Feng-Rui
FR
Wang
Xue-Ming
XM
Yang
Xing-Yi
XY
Jin
Lai-Xiang
LX
Lin
Yi-Na
YN
Sun
Zu-Pei
ZP
Chen
D000293:Adolescent; D000328:Adult; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D007455:Iodine / Q000097:blood*; D011247:Pregnancy; D011264:Pregnancy Trimesters / Q000097:blood; D012016:Reference Values; D013961:Thyroid Gland / Q000378:metabolism*; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood; D014284:Triiodothyronine / Q000097:blood; D055815:Young Adult
D014284:Triiodothyronine; D013972:Thyrotropin; D007455:Iodine; D013974:Thyroxine