Endocrine Abstracts

The reference ranges of thyroid function tests during pregnancy are still being debated, even in ethnically homogenous populations. The defining of the normal range is of importance, as it influences therapeutic decisions, particularly the use (or over-use) of levothyroxine in this vulnerable population. The study was aimed at the longitudinal assessment of thyroid function tests results in marginally iodine sufficient Polish population of pregnant women. The study was performed between 2007-2017 as a part of the Polish National Programme for Elimination of Iodine Deficiency. The study included 1405 pregnant women (222 – 1st trimester, 561 – 2nd trimester, 622 – 3rd trimester of pregnancy) of median age 29 years (IQR - 6 years). In each woman serum TSH, FT4, FT4 and aTPO, as well as urinary iodine concentration (UIC) in a urine spot sample, were measured.

Results: Median TSH was, respectively: 1st trimester – 1.22 mIU/l (IQR – 1.42 mIU/l; 2.5–97.5 percentile: 0.04–4.02 mIU/l); 2nd trimester – 1.63 mIU/l (IQR – 1.28 mIU/l; 2.5–97.5 percentile: 0.19–4.42 mIU/l), 3rd trimester – 1.61 mIU/l (IQR – 1.13 mIU/l; 2.5-97.5 percentile: 0.3–4.28 mIU/l). There was no significant yearly difference in TSH concentrations. Median FT4 was, respectively: 1st trimester 15.19 pmol/l (IQR-2.37 pmol/l; 2.5–97.5 percentile: 11.65-21.68 pmol/l), 2nd trimester - 12.69 pmol/l (IQR-2.61 pmol/l; 2.5p–97.5 percentile: 9.29-17.32 pmol/l), 3rd trimester – 11.95 pmol/l (IQR -2.97 pmol/l; 2.5-97.5 percentile: 8.56–17.26 pmol/l). ATPO positivity was found in 18%, 15%, and 9% of pregnant women in the 1st, 2nd and 3rd trimester, respectively. Median TSH in aTPO-negative women was, respectively: 1st trimester – 1.11 mIU/l (2.5-97.5 percentile: 0.03–3.75 mIU/l), 2nd trimester 1.57 mIU/l (2.5-97.5 percentile: 0.18-4.18 mIU/l), 3rd trimester 1.60 mIU/l (2.5-97.5 percentile: 0.35-4,23). ATPO negative and positive women in the 1st and 2nd trimester of pregnancy differed significantly in mean TSH concentrations (1st trimester: 1.31 vs 2.24 mIU/l, P=0.002; 2nd trimester: 1.73 vs 2.25 mIU/l, P=0.001; 3rd trimester 1.78 vs 2.02 mIU/l, P=0.230). The significant difference in FT4 according to aTPO status was found only for the 3rd trimester (P=0.020). No difference was found between those groups in FT3 and UIC concentrations. The stepwise regression model failed to find significant relation between TSH and pregnancy week, aTPO positivity, and UIC.

Conclusions: Our results support the view that the upper TSH range in pregnancy is only slightly lower than in a general population. Therefore, more caution is needed while diagnosing hypothyroidism and deciding on treatment with levothyroxine during gestation.