Thyroid-Stimulating Hormone Reference Ranges in the First Trimester of Pregnancy in an Iodine-Sufficient Country

Abstract

BackgroundThyroid dysfunction is associated with negative neonatal and obstetric outcomes. Large differences in thyroid function reference intervals exist across different populations. These differences can be explained by population-specific factors, such as iodine status. Many countries in Latin America report iodine sufficiency, but relatively few countries have published up-to-date data on iodine levels and thyroid function in the overall population, and especially in pregnant women. We evaluated the iodine status of pregnant women in Chile and determined thyroid hormone reference ranges in this population.MethodsThis was a prospective observational study of healthy Chilean women at their first prenatal visit before week 14. Thyroid-stimulating hormone (TSH), total thyroxine (T4), free T4, antithyroid peroxidase antibody (TPOAb), and iodine levels from spot urine samples were measured. Iodine status and the reference ranges for TSH were calculated.ResultsA total of 1,022 pregnant women in the first trimester were selected. Urinary iodine levels were measured in 302 randomly-selected women. The median urinary iodine concentration was 173.45 µg/L (interquartile range, 108.11 to 249.35).The reference ranges of TSH were calculated in 670 patients selected according to the National Academy of Clinical Biochemistry guidelines. The median TSH level was 1.88 µIU/mL (2.5th percentile: 0.13 to 97.5th percentile: 5.37). Using the reference range in the 1,022 women, the prevalence of clinical hypothyroidism was 1.76%, and that of subclinical hypothyroidism was 3.92%. TPOAb positivity was more common in women with TSH levels above 3.5 µIU/mL.ConclusionWe found adequate iodine intake and a right-shifted distribution of serum TSH levels in pregnant women in Chile. The prevalence of hypothyroidism in our sample of pregnant women was higher than has been described in the literature.

Thyroid-Stimulating Hormone Reference Ranges in Early Pregnancy: Possible Influence of Iodine Status

“…Therefore, the prevalence of subclinical and/or overt hypothyroidism was 34.8%, 10.4%, or 5.7% according to the reference range suggested by the 2011 ATA guideline, the manufacturer's reference, and the present study, respectively (shown in Fig. 3 of Castillo et al [ 12 ]). This discrepancy means that the number of pregnant women to be treated by levothyroxine varies significantly according to the choice of the TSH reference range.…”

“…In this issue of Endocrinology and Metabolism , Castillo et al [ 12 ] determined the thyroid hormone reference range of pregnant women in Chile. They recruited 1,022 pregnant women in their first trimester, checked their thyroid function and TPOAb status, and used NACB guidelines to select 670 subjects as the reference population.…”

“…Castillo et al [ 12 ] also evaluated the iodine status of pregnant women in Chile. They stated that the Chilean universal salt iodine fortification program started in 1979, and required each kilogram of salt to be fortified with 100 µg of iodine (100 ppm).…”

“…Nonetheless, some proportion of the population still showed iodine deficiency or more than adequate iodine status, as shown in Fig. 2 of the paper by Castillo et al [ 12 ]. Those historical changes in the iodine fortification program and/or current iodine status might explain why the population of their study showed a right-shifted distribution of serum TSH during pregnancy.…”

Response: Thyroid-Stimulating Hormone Reference Ranges in the First Trimester of Pregnancy in an Iodine-Sufficient Country (Endocrinol Metab 2018;33:466–72, Carmen Castillo et al.)

“…In 2016, we reported that body mass index (BMI) influenced pregnancy thyroid hormone reference ranges in Chilean women, we found that TSH distribution was shifted towards higher values and free thyroxine towards lower values in obese pregnant women compared to normal weight pregnant women [4], now we are reporting the influence of iodine [5].…”