Thyroid Function During Pregnancy in A Multiethnic Population in Norway

Context

Ethnic differences in thyroid function during pregnancy have been reported. However, it is unclear if this is equally valid across ethnic groups within multiethnic populations.

Objective

We aimed to assess ethnic differences in thyrotropin (TSH) and free thyroxine (FT4), and the prevalence of thyroid dysfunction and thyroid autoimmunity during pregnancy.

Methods

In a population-based cohort of 785 pregnant women in Oslo, Norway, TSH, FT4, and thyroid peroxidase antibodies (TPO Abs) were measured twice: at gestational week (GW) 15 and 28, and urine iodine concentration at GW 15. Associations were assessed using multivariate linear regression.

Results

We found ethnic differences in TSH levels at both time points, but not for fT4. South Asians had 0.42 mU/L (95% CI, 0.20-0.64) higher TSH than Europeans in GW 15. This difference persisted after adjusting for covariates (including TPO Ab positivity and iodine status), and increased further as pregnancy progressed. In contrast, East Asians had the lowest TSH. No new cases of overt hypothyroidism were detected in early pregnancy, but subclinical hypothyroidism was found in 6.6% among all, highest in South Asians (14.2%). Hyperthyroidism early in pregnancy was observed in 3.7% (almost all subclinical), highest in East Asians (11.9%). The prevalence of TPO Ab positivity was 4%, highest in South Asians (8%).

Conclusion

In a multiethnic population of presumably healthy women, we found ethnic variations in TSH but not FT4 levels throughout pregnancy. South Asians had higher TSH and more subclinical hypothyroidism, not explained by their higher prevalence of TPO Ab positivity. Larger studies are needed to define ethnic- and trimester-specific reference ranges in pregnancy.

Thyroid Laboratory Testing and Management in Women on Thyroid Replacement Before Pregnancy and Associated Pregnancy Outcomes

Background: Women with hypothyroidism before pregnancy often require an increase in their levothyroxine dosage to maintain a euthyroid state during pregnancy. The objectives of this study were to investigate: (i) the frequency and distribution of thyrotropin (TSH) testing and levothyroxine dosage adjustment by gestational age, (ii) the magnitude of levothyroxine increase by the underlying etiology of hypothyroidism, and (iii) the relationship of overtreatment or undertreatment during pregnancy with adverse pregnancy outcomes among women using thyroid replacement before pregnancy. Methods: A retrospective cohort study of pregnancies in women on thyroid replacement before pregnancy in Alberta, Canada, was performed. Women using thyroid replacement anytime during the two years before pregnancy who delivered between October 2014 and September 2017 were included. Delivery records, physician billing, and laboratory and pharmacy administrative data were linked. Outcomes included characteristics of TSH testing, levothyroxine dosing, and pregnancy outcomes. The frequency and gestational timing of TSH testing and levothyroxine adjustments were calculated. Multiple logistic regression was used to test whether pregnancies with TSH <0.10 mIU/L (overtreatment) or TSH ≥10.00 mIU/L (undertreatment) compared with control pregnancies (TSH 0.10-4.00 mIU/L) were associated with adverse pregnancy and neonatal outcomes. Results: Of the 10,680 deliveries, 8774 (82.2%) underwent TSH testing at least once during pregnancy, at a median gestational age of six weeks. An adjustment of levothyroxine dosage was made for 4321 (43.7%) during pregnancy. TSH in pregnancy below 0.10 mIU/L increased the odds of preterm delivery when compared with control pregnancies (adjusted odds ratio, 2.14 [95% confidence interval 1.51-2.78]). TSH ≥10.00 mIU/L during pregnancy was not associated with any adverse pregnancy or neonatal outcomes in the multivariable analysis. Conclusions: Although most women on thyroid replacement before conception had TSH measured at some point during pregnancy, it is concerning that 17.8% did not. Levothyroxine overtreatment in pregnancy was associated with preterm delivery. These findings suggest that clinicians should be careful to avoid overtreatment with levothyroxine in pregnancy.

Postpartum Thyroiditis in Women With Euthyroid and Hypothyroid Hashimoto's Thyroiditis Antedating Pregnancy

CONTEXT

Postpartum thyroiditis (PPT) is defined as the occurrence of de novo autoimmune thyroid disease accompanied by thyroid dysfunction in the first year postpartum. However, hormonal changes resembling the typical pattern of PPT have been reported to occur even in women with pregestational Hashimoto's thyroiditis (HT) on levothyroxine (LT4).

OBJECTIVE

To evaluate the risk of PPT in women with HT antedating pregnancy.

DESIGN/SETTING

Retrospective chart review of pregnant women with HT antedating pregnancy seen in a university hospital (2008-2017), who were followed from preconception up to 1 year after delivery.

PATIENTS

167 women preconceptionally diagnosed with HT and classified as hypothyroid HT (hypo-HT; n = 98) or euthyroid HT (eu-HT; n = 69), according to their thyroid status at the time of diagnosis.

OUTCOME MEASURES

PPT occurrence and associated clinical characteristics/risk factors.

RESULTS

PPT occurred in 65/167 women, with a rate statistically greater in the eu-HT than in the hypo-HT group (68.1% vs 18.4%; odds ratio [OR] 9.49, 95% confidence interval [CI] 4.62-19.49). Most of the women experiencing PPT in both groups were euthyroid at the time of first-trimester evaluation (39/47 eu-HT [83%] and 16/18 hypo-HT [88.9%]). Multivariate regression analysis showed eu-HT group and first-trimester euthyroidism to be positively associated with PPT occurrence (ORs 10.71 and 3.89, respectively).

CONCLUSION

PPT may occur in hypo-HT women on LT4 therapy, although significantly less frequently than in eu-HT women. The 4-fold higher risk of PPT in HT women maintaining euthyroidism at first -trimester of gestation suggests that the risk of PPT could be related to the amount of unaffected thyroid tissue.

A Practical Approach for the Verification and Determination of Site- and Trimester-Specific Reference Intervals for Thyroid Function Tests in Pregnancy

BACKGROUND

Population-, assay-, and trimester-specific reference intervals for thyroid function tests are necessary to assess thyroid status accurately and manage thyroid disease throughout pregnancy. This study's objective was to verify if the manufacturer's recommended trimester-specific reference intervals for thyroid tests and the American Thyroid Association's recommended total thyroxine (TT4) pregnancy reference intervals were verifiable and appropriate for use in the authors' multicultural population.

METHODS

Blood samples were obtained from the following sources: stored frozen surplus blood from women undergoing routine aneuploidy screening (first- and second-trimester samples, n = 274), women participating in an observational cohort study (second- and third-trimester samples, n = 135), and blood collected from women presenting for assessment to the labor and delivery ward (third-trimester samples, n = 35). Exclusions included thyroid medication or disease and positive thyroid peroxidase antibodies (anti-TPO). Samples were analyzed for thyrotropin (TSH), free T4 (fT4), free triiodothyronine (fT3), TT4, and anti-TPO using the Roche Cobas 8000 Modular e602 electrochemiluminescence immunoassay.

RESULTS

Nine percent of the aneuploidy screening samples were excluded prior to thyroid testing due to maternal use of thyroid medication. Six percent of analyzed samples were excluded: 5.9% with positive anti-TPO and one with a TSH >10 mIU/L. The manufacturer's recommended trimester-specific reference intervals for TSH were not verified by described standardized methods. Therefore, 95th percentile reference intervals were determined using a minimum number of samples. Reference intervals for TSH and fT4 were as follows: 9-12 weeks, 0.18-2.99 mIU/L and 11-19.2 pmol/L; second trimester, 0.11-3.98 mIU/L and 10.5-18.2 pmol/L; and third trimester, 0.48-4.71 mIU/L and 9.0-16.1 pmol/L, respectively. The TT4 reference interval after 19 weeks' gestation was 77-186 nmol/L.

CONCLUSIONS

This study provides a simple approach to verify or establish trimester-specific thyroid function reference intervals in local populations. The TT4 reference interval was lower than the interval proposed by the American Thyroid Association, suggesting the need for further study of TT4 in pregnancy and reliance on locally established fT4 reference intervals after 19 weeks, especially when there are no equivalent reference intervals for TT4.

2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum

BACKGROUND

Thyroid disease in pregnancy is a common clinical problem. Since the guidelines for the management of these disorders by the American Thyroid Association (ATA) were first published in 2011, significant clinical and scientific advances have occurred in the field. The aim of these guidelines is to inform clinicians, patients, researchers, and health policy makers on published evidence relating to the diagnosis and management of thyroid disease in women during pregnancy, preconception, and the postpartum period.

METHODS

The specific clinical questions addressed in these guidelines were based on prior versions of the guidelines, stakeholder input, and input of task force members. Task force panel members were educated on knowledge synthesis methods, including electronic database searching, review and selection of relevant citations, and critical appraisal of selected studies. Published English language articles were eligible for inclusion. The American College of Physicians Guideline Grading System was used for critical appraisal of evidence and grading strength of recommendations. The guideline task force had complete editorial independence from the ATA. Competing interests of guideline task force members were regularly updated, managed, and communicated to the ATA and task force members.

RESULTS

The revised guidelines for the management of thyroid disease in pregnancy include recommendations regarding the interpretation of thyroid function tests in pregnancy, iodine nutrition, thyroid autoantibodies and pregnancy complications, thyroid considerations in infertile women, hypothyroidism in pregnancy, thyrotoxicosis in pregnancy, thyroid nodules and cancer in pregnant women, fetal and neonatal considerations, thyroid disease and lactation, screening for thyroid dysfunction in pregnancy, and directions for future research.

CONCLUSIONS

We have developed evidence-based recommendations to inform clinical decision-making in the management of thyroid disease in pregnant and postpartum women. While all care must be individualized, such recommendations provide, in our opinion, optimal care paradigms for patients with these disorders.