The value of neonatal serum thyroglobulin determinations in the follow-up of patients with congenital hypothyroidism

Congenital Hypothyroidism due to a Low Level of Maternal Thyrotropin Receptor-Blocking Antibodies

INTRODUCTION

Maternal TSH receptor antibodies (TRAbs) can cross the placenta and affect fetal and neonatal thyroid function. Maternal TSH receptor-blocking antibodies (TBAbs) are a rare cause of congenital hypothyroidism.

CASE REPORT

Following the discovery of a highly elevated TSH on her neonatal screening test, a 10-day-old girl with no familial history of thyroid disorder was referred to the pediatric endocrinology unit. Hypothyroidism was confirmed with a highly elevated TSH (817 mIU/L, reference range 0.4-3.1) and very low levels of FT4 (1.8 pmol/L, reference range 12-22). Anti-TPO antibodies were at 81 IU/mL (reference range <34), TRAbs at 1.7 IU/L (reference range <1.75), and thyroglobulin at 9.4 µg/L (reference range 3.5-77). The thyroid appeared normal on ultrasonography, and no radioiodine uptake was seen on the scintigraphy after the perchlorate discharge test. Concomitantly, a severe maternal hypothyroidism was discovered (TSH 224 mIU/L). The maternal ultrasound appeared normal, anti-TPO antibodies were moderately elevated, and TRAbs were at 3.2 IU/L. TBAbs activity was measured in the mother and her daughter, and a very high and similar blocking activity was observed in both patients (TBAbs 89%, reference range <10%). L-thyroxine treatment was introduced in the newborn and was successfully discontinued at 6.5 months of age, as the TBAbs activity decreased.

CONCLUSION

We report herein a case of transient congenital hypothyroidism with a normal neonatal TRAbs level. In case of maternal TBAbs, similar activity of maternal TBAbs must be expected in the neonate, independently of the neonatal level of TRAbs.

Can we interpret serum thyroglobulin results?

Thyroglobulin (Tg) is a tumour marker for differentiated thyroid cancer. Interpretation requires a knowledge of the current thyrotropin (TSH) concentration as secretion is TSH-dependent. While a raised serum Tg may be indicative of residual or recurrent thyroid cancer, trauma to the thyroid (e.g. surgical, biopsy or due to radioiodine treatment) also causes an increase. Tg may be measured when TSH is suppressed and also following recombinant TSH (rhTSH) stimulation. Interpretation of results in pregnancy and in children is discussed. Assay bias and interference by endogenous Tg antibodies (Abs) are the main confounders in the interpretation of results. Although there is an international standard for Tg, there are large differences in results and yet there are few assay-specific clinical decision limits. Patients should therefore be monitored with the same assay. Endogenous TgAbs may cause false-negative interference in immunometric assays and may cause false-positive results in radioimmunoassay. Although the measurement of TgAbs has been advocated for predicting interference, it is now clear that interference can still occur when TgAbs have not been detected, the effect being TgAb-assay-specific. Approaches to identifying those samples where there may be interference are discussed. The laboratory should have a protocol for the investigation of possible interferences and data on the bias of the Tg assay that they use. An appreciation of the clinical uses of the service is required as an understanding by endocrinologists, oncologists and endocrine surgeons of the analytical limitations of the service.

The hypothalamic-pituitary-thyroid negative feedback control axis in children with treated congenital hypothyroidism

Measurements of serum concentrations of free T4, T3, TSH, and thyroglobulin (Tg) were conducted in 42 infants (2-9 months of age) detected and treated through the Northwest Newborn Regional Screening Program and 63 children and adolescents (1-18 yr of age) with congenital hypothyroidism (CH) detected and managed in the Northern California Kaiser Permanente Medical Care Program. Normal feedback control axis data were developed by Quest Diagnostics, Inc. - Nichols Institute Diagnostics and Loma Linda University, from free T4 and TSH measurements in 589 healthy subjects, 2 months to 54 yr of age; 83 untreated hypothyroid patients; and 116 untreated hyperthyroid patients. Twenty-four of the 42 CH infants and 57 of the 63 CH children manifested serum TSH concentrations appropriate for the measured free T4 level. In the remaining 18 infants and 6 children, serum free T4 values were increased 0.2-1.4 ng/dL (2.6-18.0 pmol/L) for the prevailing TSH level, suggesting a state of mild to moderate pituitary-thyroid hormone resistance. In the treated children, the mean T3 concentration was lower (by 32%, 102 vs. 150 ng/dL; 1.57 vs. 2.31 nmol/L) than in normal children, in agreement with earlier data in hypothyroid adults treated with exogenous T4. Serum Tg concentrations were normal or elevated in 90% of the 19 children with ectopic glands and 93% of 27 children with eutopic glands in whom measurements were available. There was a positive correlation between serum TSH and Tg concentrations (P < 0.001), suggesting significant endogenous thyroid hormone production in these children. Our results suggest that the majority of infants and children with CH have a normal hypothalamic-pituitary-thyroid negative feedback control axis during treatment and that the measurement of serum TSH is a useful marker complementing the free T4 measurement in the management of children with CH. A minority have variable pituitary-thyroid hormone resistance, with relatively elevated serum TSH levels for their prevailing serum free T4 concentration. The prevalence of resistance is greater (43%) in young infants (< 1 yr of age) than in older children (10%), indicating that, in most children, the resistance improves with age.

Apparent congenital athyreosis contrasting with normal plasma thyroglobulin levels and associated with inactivating mutations in the thyrotropin receptor gene: are athyreosis and ectopic thyroid distinct entities?

Loss-of-function mutations in the TSH receptor gene (TSH-R), usually leading to asymptomatic hyperthyrotropinemia, have been reported since 1995 in a total of eight pedigrees, with a pattern of transmission suggesting autosomal recessive inheritance. Although normal TSH secretion and action are not necessary for normal migration of the thyroid analage, they are essential for normal thyroid growth and function. In keeping with this concept, we report a severely hypothyroid boy with a normally located but very hypoplastic and hypofunctional thyroid caused by TSH-R loss-of-function mutations. The propositus' maternal great aunt also had apparent athyreosis. The propositus had undetectable uptake on 99mpertechnetate scintigraphy but normal plasma thyroglobulin at 15 days of age. He was found to be a compound heterozygote for TSH-R mutations, with the maternal allele carrying a splicing mutation (G to C transversion at position +3 of the donor site of intron 6) and the other allele a deletion of two nucleotides (2 bases of codon 655 in exon 10). The great aunt's TSH-R was normal. We also report the sex ratio of hypothyroid newborns referred to our center since 1989 with apparent athyreosis (5 girls, 7 boys) and with ectopic thyroid tissue (41 girls, 15 boys). We conclude that different genetic and nongenetic mechanisms for athyreosis and ectopic thyroid are likely, and that these two distinct entities are themselves heterogeneous. Our results further show that inactivating mutations in TSH-R may account for some cases of apparent congenital athyreosis and should be suspected, especially if plasma thyroglobulin levels are normal.