TSH receptor and Gs(alpha) genetic analysis in children with Down's syndrome and subclinical hypothyroidism

Leucine Rich Repeat Proteins: Sequences, Mutations, Structures and Diseases

Mutations in the genes encoding Leucine Rich Repeat (LRR) containing proteins are associated with over sixty human diseases; these include high myopia, mitochondrial encephalomyopathy, and Crohn's disease. These mutations occur frequently within the LRR domains and within the regions that shield the hydrophobic core of the LRR domain. The amino acid sequences of fifty-five LRR proteins have been published. They include Nod-Like Receptors (NLRs) such as NLRP1, NLRP3, NLRP14, and Nod-2, Small Leucine Rich Repeat Proteoglycans (SLRPs) such as keratocan, lumican, fibromodulin, PRELP, biglycan, and nyctalopin, and F-box/LRR-repeat proteins such as FBXL2, FBXL4, and FBXL12. For example, 363 missense mutations have been identified. Replacement of arginine, proline, or cysteine by another amino acid, or the reverse, is frequently observed. The diverse effects of the mutations are discussed based on the known structures of LRR proteins. These mutations influence protein folding, aggregation, oligomerization, stability, protein-ligand interactions, disulfide bond formation, and glycosylation. Most of the mutations cause loss of function and a few, gain of function.

Subclinical hypothyroidism in childhood - current knowledge and open issues

Subclinical hypothyroidism is defined as serum levels of TSH above the upper limit of the reference range, in the presence of normal concentrations of total T₄ or free T₄. This biochemical profile might be an indication of mild hypothyroidism, with a potential increased risk of metabolic abnormalities and cardiovascular disease recorded among adults. Whether subclinical hypothyroidism results in adverse health outcomes among children is a matter of debate and so management of this condition remains challenging. Mild forms of untreated subclinical hypothyroidism do not seem to be associated with impairments in growth, bone health or neurocognitive outcome. However, ongoing scientific investigations have highlighted the presence of subtle proatherogenic abnormalities among children with modest elevations in their TSH levels. Although current findings are insufficient to recommend levothyroxine treatment for all children with mild asymptomatic forms of subclinical hypothyroidism, they highlight the potential need for assessment of cardiovascular risk among children with this condition. Increased understanding of the early metabolic risk factors associated with subclinical hypothyroidism in childhood will help to improve the management of affected individuals.

Hypothyroidism in preterm infants following normal screening

Congenital hypothyroidism is screened for in the UK using blood spot thyroid-stimulating hormone (TSH) screening at 5-8 d of age. Although standards are set by the UK Newborn Screening Programme Centre, there are variations in TSH cut-offs used. The introduction of repeat screening of preterm babies at 36 weeks' gestational age in 2005 was controversial in its utility and timing. Two cases of preterm babies are presented, who had normal blood spot TSH values on the first test and who became screen positive when re-tested at term. The first with Trisomy 21 was born at 29 + 6 weeks with an initial blood spot TSH of 3.3 mU/L rising to 263 mU/L at term-corrected gestational age (plasma TSH 476.5 mU/L). The second was born at 24 + 6 weeks' gestational age and on day 7, the heel prick blood spot TSH was <2 mU/L, rising to 6.4 mU/L at 36 weeks corrected gestational age. After a barium enema, the plasma TSH increased to 66.6 mU/L with a free thyroxine of 7.6 pmol/L at day 101. Both cases were treated with thyroxine until death due to complications of prematurity. These cases illustrate the difficulties in screening for congenital hypothyroidism in preterm infants, due to the immaturity of the hypothalamo-pituitary-thyroid axis, and the effect of intercurrent illness and drugs on thyroid function. Despite a reassuring published review of 2200 preterm infants, these cases suggest that it may be unwise not to re-screen ex-preterm infants for congenital hypothyroidism at term.

TSH Isoforms: About a Case of Hypothyroidism in a Down's Syndrome Young Adult

Background. For unknown reasons, the prevalence of thyroid autoimmune disorders is higher in patients with Down's syndrome than in the general population. The present case strongly supports a recent evaluation of propagating screening for thyroid disease in this group of patients to assure early diagnosis of hypothyroidism. Methods. In a 25-year-old man diagnosed with Down's syndrome, clinical manifestations of hypothyroidism were lacking, but profound biochemical abnormalities were found with particularly high levels of thyroid stimulating hormone (TSH). Antigenic properties of TSH were characterized using a panel of anti-TSH antibodies. Results. Technical problems not infrequently associated with TSH measurements are convincingly ruled out. Antigenic characterization of the patient's circulating TSH revealed circulating forms of TSH different from pituitary TSH which closely resembled TSH recombinant human hormone. Conclusions. It appears counterintuitive that the bioactivity of TSH decreases in the hypothyroid state as higher bioactivity of TSH is anticipated in hypothyroidism promoted by an increased hypothalamic TRH drive. In contrast, diminished negative thyroid hormone feedback will enhance posttranslational glycosylation of TSH subunits and increase sialylation of the carbohydrate side chains. Both exert a negative effect on TSH bioactivity, only compensated by the very high levels of the hormone as in the present case.

Subclinical hypothyroidism in children: normal variation or sign of a failing thyroid gland?

Subclinical hypothyroidism (SCH), defined by a normal total or free T4 level and a mildly elevated TSH (typically 5–10 mU/L), is common in children, but there is currently no consensus on management. Several recent pediatric studies indicate that progression of SCH to overt hypothyroidism (OH) is uncommon and that over a period of several years, elevated TSH usually either normalizes or persists but does not increase. The etiology appears to be multifactorial, with some cases representing minor developmental abnormalities, some related to obesity, some to mild autoimmune thyroiditis, and some associated with mutations in the gene for the TSH-receptor. There are no pediatric studies showing clinical benefit of treating these children with thyroid hormone, but additional studies in this area are needed. Since few cases of pediatric SCH progress to OH, treatment can be deferred, and periodic follow-up testing may be the preferred strategy, with elevated thyroid antibodies or a goiter being considered risk factors for eventual OH.

TSH-Dependent expression of the LDL receptor-associated protein (RAP) in thyroid epithelial cells

The low density lipoprotein (LDL) receptor-associated protein (RAP) is an endoplasmic reticulum (ER)-resident molecular chaperone for several LDL receptor family members and it also binds to thyroglobulin (Tg), the thyroid hormone precursor. Disruption of the RAP gene in thyrocytes results in impaired Tg secretion. To gain further insights into the function of RAP in the thyroid, we investigated whether its expression in thyrocytes is regulated by thyroid-stimulating hormone (TSH), a feature common to all proteins involved in thyroid hormone secretion. We found by immunofluorescence that in FRTL-5 cells cultured in the presence of TSH, RAP is expressed intracellularly. The levels of expression increased after exposure to TSH, beginning at 48 hours, in a concentration-dependent manner as observed by immunofluorescence and Western blotting. Expression of RAP was also increased by TSH in primary cultures of human thyrocytes as observed by Western blotting. In hypothyroid mice with high serum TSH, RAP was markedly increased compared with euthyroid mice as observed by immunohistochemistry and Western blotting. Based on these findings, we concluded that RAP is expressed by thyrocytes in a TSH-dependent manner, both in cultured thyroid cells and in vivo.

Trisomy 21 causes persistent congenital hypothyroidism presumably of thyroidal origin

OBJECTIVE AND DESIGN

Lowered neonatal plasma thyroxine (T(4)) and mildly elevated thyrotropin concentrations together with developmental benefits from neonatally started T(4) treatment in a randomized clinical trial demonstrated Down syndrome (DS) neonates to be mildly hypothyroid, at least during their first weeks of life. To prove that this hypothyroid state persists beyond this period in all, and to elucidate its etiology, we evaluated the course of the thyroid function determinants in all DS infants participating in this 24-month trial.

MAIN OUTCOME

Mean plasma thyrotropin concentrations and thyrotropin frequency distributions of 97 placebo-treated infants were persistently shifted to substantially higher concentrations, while free T(4) frequency distributions were in the lower two thirds of the reference interval. Mean thyroglobulin concentrations were normal. To normalize plasma thyrotropin, T(4)-treated DS infants (N = 99) needed rather high free T(4) concentrations, like T(4)- treated non-DS children with thyroidal congenital hypothyroidism. At ages 12 and 24 months, thyroid peroxidase antibodies were detected in 1.1% and 5.4% of all DS infants.

CONCLUSIONS

These findings suggest that as a group DS infants have a novel type of persistent mild congenital hypothyroidism, presumably of thyroidal origin. The group character suggests a direct relation with the trisomic state of chromosome 21, hypothetically through genomic dosage imbalance of dosage-sensitive genes interfering with thyroid hormone production.