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?1

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 anlage, 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.

Comparative plasma thyroglobulin measurements with three non-cross-reactive monoclonal antibodies in metastatic thyroid cancer patients

In 12 normal individuals and 25 patients with metastases of differentiated thyroid cancer, plasma thyroglobulin (Tg) concentrations were measured simultaneously with three immunoradiometric assays. Each of the three systems used a different, non-cross-reactive monoclonal Tg antibody as second antibody. In general, there was a very close correlation between the results of the three different systems in the cross-sectional study of the 25 cancer patients as well as in longitudinal follow-up studies in selected patients. The monoclonal antibody Tg40 produced values about 30% higher than the two other systems. The difference was, however, not statistically significant owing to the large scatter. The monoclonal antibody Tg 13 appeared to be very sensitive to interference with thyroglobulin auto-antibodies. In conclusion, the monoclonal antibodies against the three epitopes tested produced very similar Tg values in normal individuals and 25 patients with metastatic thyroid cancer; however, before more is known about epitope specificity of Tg autoantibodies and heterogeneity of tumour Tg, monoclonal antibodies should be used for routine measurements only with caution.

Concentration of plasma thyroglobulin and urinary excretion of iodinated material in the diagnosis of thyroid disorders in congenital hypothyroidism

In this paper we describe methods for the early aetiological diagnosis of congenital hypothyroidism, using beside the classical T4, T3 and TSH plasma concentrations, four additional parameters in plasma and urine. The first one is thyroglobulin (Tg). In normal children of more than one year of age and in adults, 5–35 ng/ml plasma is found, in neonates 2–3 weeks old, this level is 10–250 ng/ml. In patients with a stimulated thyroid gland, as in primary congenital hypothyroidism, plasma Tg levels increase. High Tg values are found in iodine deficiency and in organification defects. In the absence of the thyroid gland plasma Tg is undetectable. Low to normal levels are found in cases with hypoplasia of the gland. In patients with a disturbed synthesis of Tg, resulting in Tg deficiency of the gland, plasma Tg levels vary from undetectable to normal. The PBI-T4 plasma difference, which is caused by circulating abnormal iodoproteins is the second parameter. The products of thyroidal breakdown processes of the abnormal iodoproteins are excreted in the urine and used as the third parameter. We found that the excretion of this low molecular weight iodinated material (LOMWIOM) was increased only in Tg-deficient patients. If the neonate is found to be hypothyroid, thyroid hormone substitution must be given immediately. Blood and urine sampling can be done just before or even directly after starting the therapy. The measurements extended with the determination of the total iodine excretion (fourth parameter) can be carried out within 1 week. With these additional methods it appeared to be possible to distinguish between several types of congenital hypothyroidism in neonates found by screening.