Endocrinol Metab.  2018 Dec;33(4):466-472. 10.3803/EnM.2018.33.4.466.

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

• Castillo C
• Lustig N
• Margozzini P
• Gomez A
• Rojas MP
• Muzzo S
• Mosso L

• KMID: 2427442
• DOI: http://doi.org/10.3803/EnM.2018.33.4.466

Abstract

BACKGROUND
Thyroid 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.
METHODS
This 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.
RESULTS
A 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.
CONCLUSION
We 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.

Keyword

Pregnancy; Hypothyroidism; Thyroid; Iodine

MeSH Terms

Biochemistry
Chile
Female
Humans
Hypothyroidism
Iodine
Latin America
Observational Study
Peroxidase
Pregnancy
Pregnancy Trimester, First*
Pregnancy*
Pregnant Women
Prevalence
Prospective Studies
Reference Values*
Thyroid Gland
Thyrotropin*
Thyroxine
Iodine
Peroxidase
Thyrotropin
Thyroxine

Figure

• 

  Fig. 1 Flow diagram of patients. TPOAb, antithyroid peroxidase antibody; fT4, free thyroxine.

• 

  Fig. 2 Urinary iodine levels during the first trimester of pregnancy. Median urinary iodine level: 173.45 µg/L.

• 

  Fig. 3 Distribution of thyroid-stimulating hormone (TSH) levels (µIU/mL) in the first trimester of pregnancy in 1,022 women and differences in the prevalence of hypothyroidism using different cut-off points (2.5, 4.2, and 5.36 µIU/mL).

• 

  Fig. 4 Prevalence of positive antithyroid peroxidase antibody (TPOAb) according to thyroid-stimulating hormone (TSH) levels in pregnant women during the first trimester. aSignificant differences of anti-TPOAb prevalence with respect to the lowest prevalence within the normal range (5.66%; 95% confidence interval [CI], 2.03 to 9.29) in 1.51 to 2.0 TSH category. Prevalence in 3.01 to 3.50, 3.51 to 4.0, 4.01 to 4.50, 5.01 to 7.50, 7.51 to 10.00, and 10.01+; TSH groups are 13.16% (95% CI, 5.38 to 20.93), 17.02% (95% CI, 5.87 to 28.18), 22.22% (95% CI, 5.46 to 38.98), 34.04% (95% CI, 19.98 to 48.11), 46.15% (95% CI, 14.8 to 77.51), and 46.15% (95% CI, 14.8 to 77.51), respectively.

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