Increased TBG during pregnancy and increased hormonal requirements

Two logistic models for the prediction of hypothyroidism in pregnancy

BACKGROUND

The mounting evidence linking hypothyroidism during pregnancy with poor pregnancy outcome underscores the need for screening and, therefore, a search for more reliable and cheaper screening methods.

METHODS

The study was conducted in two phases. The phase one study comprised of healthy women in different stages of pregnancy who attended routine antenatal clinic at St Theresa's Maternity Hospital, Enugu, Nigeria from September 6 to October 18 1994. In this study the variables compared between the hypothyroid and non-hypothyroid pregnant women were maternal age, the number of the pregnancy or gravidity, gestational age, social class, body weight, height, the clinically assessed size of the thyroid gland, serum free thyroxin (FT4) and serum thyrotrophin (TSH). Based on the parameter differences between the two comparison groups of pregnant women two Logistic models, Model I and Model 11, were derived to differentiate the hypothyroid group from their non-hypothyroid counterparts. The two logistic models were then applied in a prospective validation study involving 197 pregnant women seen at presentation in Mother of Christ Specialist Hospital and Maternity, Ogui Road, Enugu from March 2002 to November 2007

FINDINGS

The findings were that 82 (50.3%) of the 163 pregnant women had thyroid gland enlargement while 60 (36.8%) had hypothyroidism as defined by FT4 values below and/or TSH above their laboratory reference ranges. The pregnant subjects with hypothyroidism, compared with their non-hypothyroid counterparts, were characterized by a higher gravidity (p < 0.01), a higher body weight (p < 0.01), a higher goiter prevalence rate (p < 0.01) and a more advanced gestational age (p < 0.0001). A significant, positive correlation was also found between body weight and gestational age (r = 0.5; p < 0.01) At the cut-off point for Model l (fitted with gravidity, thyroid size and gestational age) it had a sensitivity of 100%, a specificity of 72.8% and an overall predictive accuracy of 82.9%; whereas for Model II (fitted with gravidity, thyroid size and body weight) the sensitivity was 100%, the specificity was 59.2% and the overall accuracy of discrimination was 74.8%. In the prospective validation study both models showed a sensitivity of 100% each with specificities of 85.5% for Model I and 76.2% for Model II.

CONCLUSION

It is concluded that logistic models fitting gravidity, thyroid gland size and gestational age or body weight are useful alternatives in screening for hypothyroidism during pregnancy. There is, however, a need for further independent confirmation of these findings.

Clinical Controversies in Screening Women for Thyroid Disorders During Pregnancy

Subclinical hypothyroidism and/or the presence of thyroid peroxidase antibodies (TPOAb) may be associated with subfertility, infertility, spontaneous abortion, placental abruption, preterm delivery, gestational hypertension, preeclampsia, postpartum thyroid dysfunction, depression (including postpartum depression), and impaired cognitive and psychomotor child development. In November 2002, the American Association of Clinical Endocrinologists (AACE) released new guidelines for clinical practice for the diagnosis and treatment of hyperthyroidism and hypothyroidism, which includes a new thyroid-stimulating hormone (TSH) reference range of 0.3 to 3.0 mIU/L. Recently, the AACE recommended screening all women considering conception and/or all gravid women in the first trimester for thyroid dysfunction. However, the American College of Obstetricians and Gynecologists (ACOG) and the United States Preventive Services Task Force (USPSTF) have not endorsed these recommendations. This article reviews the evidence regarding screening women during pregnancy for subclinical hypothyroidism and/or the presence of thyroid peroxidase antibodies.

Therapeutic drug monitoring during pregnancy and lactation: thyroid function assessment in pregnancy-challenges and solutions

The diagnosis and monitoring of thyroid disease necessitates the knowledge of thyroid pathophysiology and of the technical limitations of current thyroid-related biochemical tests. Thyroid disease diagnosis and monitoring are further complicated during pregnancy and lactation, due to pregnancy-related changes in thyroid hormone metabolism. Dramatic changes that occur in thyroxine and triiodothyronine ranges during pregnancy pose challenges for hypothyroid gravidas. Very early in pregnancy, levothyroxine replacement needs to be increased. Moreover, increases in thyroid hormone replacement need to be conducted individually and on a timely basis. For reasons that are still not entirely clear, although dependent in part on changes in thyroxine binding, free thyroxine (FT4) levels decrease as pregnancy progresses necessitating the use of trimester-specific reference intervals for appropriate replacement. Thyroxine binding protein levels vary by hormonal status, inheritance, and disease states and are higher in pregnancy; hence, FT4 assays became popular because they measure the unbound hormone. However, current FT4 immunoassays are estimate tests that do not reliably measure FT4 and are known to be sensitive to alterations in binding proteins and therefore are method-specific. The need to reliably identify hypothyroxinemic pregnant patients, especially in the first trimester, is of prime importance for early fetal brain development before the fetal thyroid functions. This article addresses 1) the current limitations of laboratory-free thyroxine immunoassay methodologies and especially during pregnancy; 2) trimester-specific reference intervals for thyroid function tests; and 3) the study of levothyroxine pharmacokinetics in pregnant and nonpregnant women.

Thyroid function tests during first-trimester of pregnancy: a review of literature

This literature review was conducted to summarize the main points of maternal thyroid function tests, with particular attention in the first trimester of pregnancy which accompanied with significant biochemical and metabolic alteration. The evaluation of thyroid function of either hyperthyroidism and hypothyroidism should be assessed by determination of serum Thyroid Stimulating Hormone (TSH), Thyroxine (T4), Triiodothyronine (T3), Iodine and Thyroid Autoantibodies. Glomerular filtration rate is increased during pregnancy; therefor iodine deficiency should be evaluated during the pregnancy to prevent hypothyroidism. The role which can be played by Human Chronic Gonadotropin (hCG) on stimulating the thyroid gland to become over-active was investigated. Serum level ofthyroglobulin (Tg) and Thyroxin Binding Globulin (TBG) should be assessed for proper assessments of thyroid gland during pregnancy. Thyroid function tests during first-trimester of pregnancy and particularly the reference interval for thyroid function tests for pregnant women in each region has to be established, to prevent mis-diagnosis and irreversible mental and physical adverse affect for growing fetus.

The magnitude of increased levothyroxine requirements in hypothyroid pregnant women depends upon the etiology of the hypothyroidism

BACKGROUND

In the United States, many women with hypothyroidism are on thyroid hormone replacement during pregnancy. The optimal management strategy for thyroid hormone dosing in hypothyroid women during pregnancy is controversial. We hypothesized that dosage requirements during pregnancy might differ depending upon the nature of the underlying hypothyroidism.

METHODS

We conducted a retrospective review of 45 pregnancies from 38 women whose hypothyroidism was managed during pregnancy. Thyroid function tests were obtained when pregnancy was confirmed, then every 4-8 weeks. The thyrotropin (TSH) goal was 0.4-4.1 microU/mL (SI unit conversion: multiply TSH by 1.0 for mIU/L).

RESULTS

On average, the entire group required a cumulative increase from baseline in levothyroxine (LT(4)) dosage of 13% in the first trimester, 26% in the second trimester, and 26% in the third trimester (p < 0.001, p < 0.001, p < 0.001, respectively). Average baseline LT(4) dose for patients with primary hypothyroidism was 92.5 +/- 32.0 microg daily. These patients required small cumulative dose increases of 11%, 16%, and 16% from baseline in each trimester, respectively (p values = 0.125, 0.016, 0.016). Average baseline LT(4) dose for patients with hypothyroidism resulting from treated Graves' disease or goiter was 140.4 +/- 62.4 microg daily. These patients required the largest cumulative increases in LT(4) dosage (first trimester, 27%; second trimester, 51%; third trimester, 45%; p = 0.063, 0.063, 0.063, respectively). Average baseline LT(4) dose for patients with thyroid cancer was 153.2 +/- 30.3 microg. The cumulative LT(4) dose increases for patients with thyroid cancer were 9%, 21%, and 26% in each trimester, respectively (p = 0.03, p < 0.001, p < 0.001).

CONCLUSIONS

The etiology of hypothyroidism plays a pivotal role in determining the timing and magnitude of thyroid hormone adjustments during pregnancy. Patients require vigilant monitoring of thyroid function upon confirmation of conception and anticipatory adjustments to LT(4) dosing based on the etiology of their hypothyroidism.