Endocrine Abstracts

Objective: Pregnant women worldwide are at risk for mild iodine deficiency (ID) due to transfer of iodine through placenta, increased glomerular filtration of iodine and higher maternal thyroid hormone production for foetal needs. Studies show that mild ID involves normal thyroid hormone levels but may lead to increased thyroglobulin levels resulting from an increased maternal thyroid volume through changed thyroid metabolism. When assessing thyroid hormones during pregnancy, reference ranges should preferably be based on a pregnant population. It is also internationally recommended that trimester-specific reference ranges should be established in an iodine-sufficient population, which may be difficult to achieve in countries where pregnant women typically suffer from mild ID. It is also unknown whether mild ID affects the upper and lower limits of the reference ranges. In the SWIDDICH study (www.swiddich.se, NCT02378246 clinical trials.gov), a large randomised controlled multicentre trial in Sweden, blood samples from each trimester were analysed to evaluate how trimester-specific reference ranges were affected by iodine supplementation in a mild ID pregnant population resulting in normal iodine levels (previous pilot data urinary iodine concentration (UIC) in the intervention group had UIC 136 (91-211) µg/l vs 65 (39-108) µg/l in the non-intervention group P < 0.001 n = 158).

Methods: Women without known thyroid disease, with singleton pregnancy were included during first trimester (n = 368: age range 18 – 40 years) and randomised to use a daily supplement with/without iodine (150 µg/day). Blood samples were collected at the maternal health care centre during all trimesters. TSH, free T4, free T3 and anti-TPO were measured using the Roche Cobas method; individuals with increased anti-TPO according to the reference range from the manufacturer was excluded. The limits of the reference ranges were analysed and compared between the two groups for each trimester.

Results: There was no significant difference between the two groups in the upper and lower limits of the reference ranges for FT4, FT3, TSH in any of the trimesters. There was a significant difference in all thyroid hormones between the different trimesters -except for FT3 comparing trimester 2 and 3. The reference range for FT4 (including all trimesters and both groups) were lower compared to a non-pregnant adult population.

Conclusion: Appropriate trimester-specific reference ranges are of major importance when treating women with thyroid dysfunction during pregnancy. These data shows that they can be established not only in iodine sufficient, but also in mildly ID pregnant populations. This data may impact future international guidelines and benefit healthcare for women worldwide.