The fetus in maternal hyperthyroidism

Subclinical Hyperthyroidism and Pregnancy Outcomes

OBJECTIVE

Subclinical hyperthyroidism has long-term sequelae that include osteoporosis, cardiovascular morbidity, and progression to overt thyrotoxicosis or thyroid failure. The objective of this study was to evaluate pregnancy outcomes in women with suppressed thyroid-stimulating hormone (TSH) and normal free thyroxine (fT(4)) levels.

METHODS

All women who presented to Parkland Hospital for prenatal care between November 1, 2000, and April 14, 2003, underwent thyroid screening by chemiluminescent TSH assay. Women with TSH values at or below the 2.5th percentile for gestational age and whose serum fT(4) levels were 1.75 ng/dL or less were identified to have subclinical hyperthyroidism. Those women screened and delivered of a singleton infant weighing 500 g or more were analyzed. Pregnancy outcomes in women identified with subclinical hyperthyroidism were compared with those in women whose TSH values were between the 5th and 95th percentiles.

RESULTS

A total of 25,765 women underwent thyroid screening and were delivered of singleton infants. Of these, 433 (1.7%) were considered to have subclinical hyperthyroidism, which occurred more frequently in African-American and/or parous women. Pregnancies in women with subclinical hyperthyroidism were less likely to be complicated by hypertension (adjusted odds ratio 0.66, 95% confidence interval 0.44-0.98). All other pregnancy complications and perinatal morbidity or mortality were not increased in women with subclinical hyperthyroidism.

CONCLUSION

Subclinical hyperthyroidism is not associated with adverse pregnancy outcomes. Our results indicate that identification of subclinical hyperthyroidism and treatment during pregnancy is unwarranted.

LEVEL OF EVIDENCE

II-2.

Graves' disease in pregnancy: prospective evaluation of a selective invasive treatment protocol

OBJECTIVE

Graves' disease in pregnancy carries a risk of fetal thyrotoxicosis from the transplacental transfer of thyroid-stimulating antibodies or fetal hypothyroidism from transplacental transfer of antithyroid drugs and thyroid-blocking antibodies.

STUDY DESIGN

From 1991 through 2002, all pregnant women with Graves' disease underwent follow-up evaluations that included serial thyroid-stimulating antibody level, thyroid function, and ultrasound examinations. Umbilical blood sampling was recommended if the thyroid-stimulating antibody level was abnormally high or if fetal tachycardia, goiter, intrauterine growth retardation, or hydrops were present. For fetal hyperthyroidism, the mother received antithyroid drugs; for fetal hypothyroidism, maternal antithyroid treatment was reduced, and thyroxine was injected into the amniotic sac.

RESULTS

Of 40000 deliveries, 24 pregnancies (26 fetuses) occurred in 18 women with Graves' disease. Nine of 14 mothers with positive findings elected umbilical blood sampling. In 4 of the mothers, the results were normal. Hyperthyroidism and hypothyroidism were diagnosed in 2 and 3 fetuses, respectively. All the fetuses were treated successfully by the protocol with up to four repeated umbilical blood samplings. No complications were recorded in any of the 20 umbilical blood sampling. In the 5 patients who had only elevated thyroid-stimulating antibody levels and who did not elect umbilical blood sampling, sonographic findings remained normal up to term, and the newborn infants were normal. One of 12 children (in whose case we did not recommend umbilical blood sampling) was born with transient hypothyroidism caused by maternal propylthiouracil treatment. All children, whose cases were followed for up to 9 years, are normal.

CONCLUSION

In women with Graves' disease, umbilical blood sampling in selected cases may improve the control of fetal thyroid function.

Management of fetal thyroid goitres: a report of 11 cases in a single perinatal unit

Fetal thyroid goitres may reveal hormonal imbalance. This can jeopardize neurological development and fetal outcome even when early postnatal treatment is provided. We report a series of 11 goitres diagnosed antenatally in women with past or present thyroid disorders or discovered fortuitously on ultrasound scan. Fetuses presented with hyperthyroidism in three cases and hypothyroidism in eight. Hypothyroidism was iatrogenic in five cases, due to maternal anti-thyroid drugs. Hyperthyroidism was induced by transplacental transfer of thyroid stimulating antibodies (TSHrab). Accurate diagnosis of fetal thyroid status was obtained by fetal blood sampling but this invasive method was deemed necessary only in four cases as maternal clinical and biological data and ultrasound signs provided sufficient information to infer the type of thyroid disorder in the remaining patients. Fetal therapy relied on reduction of maternal antithyroid medication and, in selected cases, intra-amniotic injection of levothyroxin in hypothyroidism, and on administration of antithyroid drugs in hyperthyroidism. All newborns were healthy and none displayed consequences of severe thyroid imbalance. No caesarean section was performed for dystocia. Fetal thyroid goitres can be managed successfully with selected use of invasive diagnostic and therapeutic techniques.

Thyroidopathies

Pregnancy affects thyroid physiology in many ways: (a) The renal iodide clearance rate is increased, hence iodine requirements increase. (b) The fetal requirements for thyroid hormones and iodide are an additional problem. (c) Serum thyroxine-binding globulin increases, thus producing an increase in the levels of total T4 and T3. (d) Chorionic gonadotropin has a thyroid-stimulating activity. This may be compensated for by a decrease in TSH, but in some cases gestational thyrotoxicosis occurs. (e) Thyroid autoimmunity usually subsides during pregnancy, but may rebound a few months after parturition, and postpartum thyroiditis may occur. Because maternal antithyroid autoantibodies cross the placenta readily, fetal and neonatal hyperthyroidism (or hypothyroidism) may develop. Pre-existing thyroid diseases are influenced. Nontoxic goiter increases in size. Iodine and/or thyroxine may be required. Graves' disease may remit. If present, antithyroid drugs should be given in small doses, and quite often they may be stopped altogether. Hypothyroid patients may require a larger T4 dose.

Fetal pituitary negative feedback at early gestational age

We describe an early prenatal diagnosis and the successful treatment of fetal Graves' disease from transplacental transfer of maternal thyroid stimulating autoantibodies (TSAb). The diagnosis of fetal thyrotoxicosis was made by umbilical cord sampling (UBS) at 20 weeks gestation, based on suppressed TSH with elevated FT4 levels. Therapy with propylthiouracil (PTU) improved fetal thyroid function tests as well as the clinical signs of fetal Graves' disease. Three more UBS were conducted before delivery indicating persisting mild fetal hyperthyroidism. Undetectable concentrations of thyrotrophin in fetal serum in the presence of markedly elevated FT4, suggests pituitary negative feedback at as early as 20 weeks gestation. Amniotic fluid thyrotrophin levels were measured at 20,24 and 26 weeks and were shown to correlate better with (elevated) maternal rather than (suppressed) fetal TSH values; therefore, we believe that amniotic fluid thyrotrophin measurement is unreliable for prediction of fetal thyroid status. Our observation is the first documentation of an intact feedback mechanism so early in fetal development and it suggests that pituitary maturation occurs earlier than previously believed.

The development of the foetal thyroid: in utero ultrasonographic measurements

OBJECTIVE

The early recognition of potentially treatable thyroid disease in the foetus frequently depends on the detection of abnormal growth of the foetal thyroid gland. We have therefore established nomograms for foetal thyroid transverse width and circumference from 14 weeks of gestation until term, using transvaginal and transabdominal high-resolution ultrasound techniques.

DESIGN

A prospective, cross-sectional study of 193 normal singleton pregnancies was performed.

MEASUREMENTS

Thyroid size was measured by transvaginal ultrasonography between 14 and 17 weeks, and by abdominal ultrasound from 18 to 37 weeks of gestation.

RESULTS

Data was accurately obtained for 193 foetuses. The mean +/- SD thyroid width and circumference were 11.7 +/- 4.1 mm (95% confidence interval 11.1-12.3) and 39.5 +/- 14.1 mm (95% confidence interval 37.4-41.6), respectively. Thyroid size as a function of gestational age was expressed by the regression equations: thyroid width (mm) = -3.94 + 0.68 x gestational age (weeks), and thyroid circumference (mm) = -1.38 + 0.23 x gestational age (weeks). The correlation coefficients, r = 0.91 and r = 0.93, for thyroid width and circumference, respectively, were found to be highly statistically significant (p < 0.0001). The normal mean of thyroid width and circumference for each week of gestational age and the 95% prediction limits were defined.

CONCLUSIONS

The present data offer normative measurements of the foetal thyroid that may facilitate the prenatal diagnosis of foetal goitre and make early administration of in utero treatment possible.

Fetal thyroid function: diagnosis and management of fetal thyroid disorders

The fetal hypothalamic-pituitary-thyroid axis develops independently of the maternal axis, but it is dependent on the maternal-placental system for adequate supply of iodide substrate. This iodide is supplied by direct transfer of maternal plasma iodide and by placental deiodination of T4. In addition, although placental transport of iodothyronines is limited, significant maternal-fetal transfer of T4 occurs, accounting for approximately 30% of the average 10 ug/dL serum-T4 concentration in fetal-cord blood at term. Current information suggests that this maternal contribution to the fetal-T4 levels is important for normal fetal maturation, particularly of the central nervous system. Combined maternal-fetal hypothyroxinemia can lead to irreversible fetal central nervous system damage. The timing of this fetal T4 dependency is not clear. It may be important in the first half of gestation, before the fetal thyroid gland is capable of T4 production, as well as the latter half of gestation when thyroid hormone effects on multiple organ systems are developing. Management of fetal thyroid dysfunction requires normalization of maternal serum T4 concentrations, avoidance or careful monitoring of potentially goitrogenic drug effects in the fetus, and in some instances, direct or indirect fetal therapy. In most cases fetal hypothyroidism is sporadic and undetected, and prognosis for normal growth and development is excellent if the mother is euthyroid and the hypothyroid state is detected and adequately treated at birth. Fetal treatment by intraamniotic thyroxine injection has been provided in cases of inadvertent maternal radioiodine treatment of Graves' disease between 10 and 20 weeks gestation and for fetal goiter detected by ultrasound. Effective treatment of fetal hyperthyroidism in pregnant women with high titers of thyroid stimulating autoantibody is possible by judicious administration of antithyroid drugs to the mother. Management of the hyperthyroid state in the neonate also is essential.

Fetal thyrotoxicosis: a case report and recommendations for prediction, diagnosis, and treatment

A maternal history of Graves' disease places the fetus at risk for thyrotoxicosis in utero via the placental transfer of thyroid-stimulating immunoglobulins. Methods for prediction of fetal hyperthyroidism are available, but are not widely used. Clinical assessment of fetal thyroid status by monitoring of fetal heart rate and growth may be inaccurate. This raises some uncertainty in the initial diagnosis of fetal thyrotoxicosis and complicates the assessment of fetal response to maternal propylthiouracil therapy. A case illustrating these pitfalls in the diagnosis and management of fetal hyperthyroidism is presented. The condition was correctly diagnosed, but treatment based on fetal heart rate resulted in biochemical hypothyroidism in the infant at birth. Current recommendations for diagnosis and treatment of fetal hyperthyroidism are reviewed along with recent developments in the field. A modified approach is proposed.