Reference Intervals for Thyroid Hormones During the First Trimester of Gestation: A Report from an Area with a Sufficient Iodine Level

Abstract

The physiological changes during pregnancy modulate the endocrine system. Therefore, both the American and the European thyroid associations recommend the use of local trimester-specific reference intervals. The purpose of this study was to establish the first trimester reference intervals for thyroid function tests in the central area of Iran. We examined 436 pregnant women in their first trimester of pregnancy, and 444 non-pregnant women in a cross sectional study. Serum levels of thyroid stimulating hormone (TSH), free thyroxin (FT4), free triiodothyronine (FT3), thyroid peroxidase antibody, urinary iodine concentration (UIC), and thyroid volume were measured for all subjects. The first trimester-specific reference intervals (2.5th–97.5th percentile) were determined for 185 pregnant women and 256 non-pregnant women with negative TPOAb, adequate iodine level (UIC≥150 μg/l in pregnant and UIC≥100 μg/l in non-pregnant women), and normal thyroid examination. We calculated multiples of the median (MoM) for TFTs to normalize the obtained data. The first trimester-specific reference intervals of serum TSH, FT4, and FT3 for pregnant women were 0.20–4.60 mIU/l, 9.0–18.02 pmol/l, and 3.40–5.64 pmol/l, respectively, while the corresponding figures for non-pregnant women were 0.59–5.60 mIU/l, 9.52–19.30 pmol/l, and 3.70–5.55 pmol/l, respectively. The first and 99th percentile MoM of TSH in pregnant women in their first-trimester was 0.06–4.62. The local normal reference ranges for the first trimester of pregnancy in central region of Iran were different from the ranges suggested by the ATA.

Reference Intervals for Thyroid Hormones During the First Trimester of Gestation: A Report from an Area with a Sufficient Iodine Level

“…Another concern was raised regarding the median (min-max) values of TSH, FT3, and FT4 for pregnant and non-pregnant women in ▶Tables 1 and ▶2 of our original paper [2] believing there is no reason that the min/max values are not in part identical.…”

“…The inclusion criteria are applied in our study to determine the reference ranges for completely healthy individuals; we excluded all women whose results showed abnormality but still did not require any treatments. However, it is known that reference ranges do not indicate how likely a particular concentration of thyroid hormone is associated with clinical disease [3]; therefore, the MoM values are suggested to be determined for all pregnant women, only excluding women with known thyroid disease at the baseline [2,3]. The MoM values and their associated centile values provide information about the likelihood of the disease for a particular concentration of the thyroid hormone in an individual to clinicians, assisting them to judge whether treatment is required.…”

“…Reference Intervals for Thyroid Hormones During the First Trimester of Gestation: A Report from an Area with a Sufficient Iodine Level A concern was raised [1] regarding the number of pregnant women in the analysis of reference range for the thyroid hormones in pregnancy [2], where we reported 185 cases and it was believed to be 145 cases.…”

“…In ▶ Fig. 1 of our paper [2], we have combined cases who were positive for TPOAb (35 + 40 = 75) and had UIC < 150 μg/l (40 + 158 = 198). Although the number of individuals whose data were analysed is correct, we rearranged the ▶Fig.…”

“…▶ Table 1 summarises participants' basic characteristics and thyroid hormone levels of 185 pregnant and 256 non-pregnant women considering exclusion criteria (outlined in page 167 in [2]) for reference range determination. However, ▶ Table 2 presents statistical data for participants with positive and negative TPOAb in pregnant and non-pregnant groups, regardless of their iodine status.…”