Thyroid function in patients with Pendred's syndrome

Clinical use of thyroglobulin: not only thyroid cancer

Thyroglobulin (TG) is a dimeric glycoprotein produced exclusively by mature thyroid tissue and stored within the follicular lumen. It is essential for the organification of iodine and the production of thyroid hormones. The concentration of TG in the bloodstream varies between individuals and depends on factors such as thyroid mass, stimulation of the gland by thyrotropin or autoantibodies, and tissue destruction. TG is essential to monitor patients with differentiated thyroid cancer; however, its use is not limited only to this clinical entity. Measurement of circulating TG can provide better insight into numerous clinical scenarios, such as destructive thyroiditis, presence of ectopic thyroid tissue, thyroid trauma, factitious thyrotoxicosis, or iodine nutrition. Lately, TG has found its new clinical use in immune checkpoint-related thyroid dysfunction. TG measurement should be performed carefully in patients with antithyroglobulin antibodies due to possible laboratory interferences. In this review, we offer a summary of current knowledge about the clinical use of TG and the implications it brings to daily practice.

Pendred syndrome

Pendred syndrome is an autosomal recessive disorder that is classically defined by the combination of sensorineural deafness/hearing impairment, goiter, and an abnormal organification of iodide with or without hypothyroidism. The hallmark of the syndrome is the impaired hearing, which is associated with inner ear malformations such as an enlarged vestibular aqueduct (EVA). The thyroid phenotype is variable and may be modified by the nutritional iodine intake. Pendred syndrome is caused by biallelic mutations in the SLC26A4/PDS gene, which encodes the multifunctional anion exchanger pendrin. Pendrin has affinity for chloride, iodide, and bicarbonate, among other anions. In the inner ear, pendrin functions as a chloride/bicarbonate exchanger that is essential for maintaining the composition and the potential of the endolymph. In the thyroid, pendrin is expressed at the apical membrane of thyroid cells facing the follicular lumen. Functional studies have demonstrated that pendrin can mediate iodide efflux in heterologous cells. This, together with the thyroid phenotype observed in humans (goiter, impaired iodine organification) suggests that pendrin could be involved in iodide efflux into the lumen, one of the steps required for thyroid hormone synthesis. Iodide efflux can, however, also occur in the absence of pendrin suggesting that other exchangers or channels are involved. It has been suggested that Anoctamin 1 (ANO1/TMEM16A), a calcium-activated anion channel, which is also expressed at the apical membrane of thyrocytes, could participate in mediating apical efflux. In the kidney, pendrin is involved in bicarbonate secretion and chloride reabsorption. While there is no renal phenotype under basal conditions, severe metabolic alkalosis has been reported in Pendred syndrome patients exposed to an increased alkali load. This review provides an overview on the clinical spectrum of Pendred syndrome, the functional data on pendrin with a focus on its potential role in the thyroid, as well as the controversy surrounding the relative physiological roles of pendrin and anoctamin.

Association of the long QT syndrome With goiter and deafness

We report on the long QT syndrome occurring in conjunction with nontoxic multinodular goiter and sensorineural deafness in several siblings of a large family. Autosomal and X-linked recessive and dominant modes of inheritance are possible for the different phenotypes. The affected family members had various phenotype combinations, suggesting variable expressivity and incomplete penetrance.

Genetic screening for deafness

Genetic testing for deafness has become a reality. It has changed the paradigm for evaluating deaf and hard-of-hearing persons and will be used by physicians for diagnostic purposes and as a basis for treatment and management options. Although mutation screening is currently available for only a limited number of genes, in these specific instances, diagnosis, carrier detection, and reproductive risk counseling can be provided. In the coming years there will be an expansion of the role of genetic testing and counseling will not be limited to reproductive issues. Treatment and management decisions will be made based on specific genetic diagnoses. Although genetic testing may be a confusing service for the practicing otolaryngologist, it is an important part of medical care. New discoveries and technologies will expand and increase the complexity of genetic testing options and it will become the responsibility of otolaryngologists to familiarize themselves with current discoveries and accepted protocols for genetic testing.

Pendred's syndrome with goiter and enlarged vestibular aqueducts diagnosed by PDS gene mutation

BACKGROUND

Pendred's syndrome (PS) is an autosomal recessive disorder characterized by goiter and congenital sensorineural hearing loss. Recent advances in molecular biology revealed the gene responsible for PS (PDS) and provided an important aid for the diagnosis of this condition.

METHODS

A case of PS with huge goiter and congenital hearing impairment was diagnosed by mutational analysis of the PDS gene.

RESULTS

Physical examination and computer tomography CT revealed a diffuse swelling of the thyroid gland. Thyroid function tests were normal, and the perchlorate discharge test was negative. Audiologic examination confirmed sensorineural hearing loss, and temporal bone CT revealed bilateral enlarged vestibular aqueducts. The mutational analysis revealed that the patient was homozygous for His 723 Arg (2168A-->G) in exon 19, a missense mutation.

CONCLUSIONS

The results of thyroid function tests in PS patients are usually normal, and the positive perchlorate discharge test has been used for the diagnosis. However, this is a nonspecific test and is not sensitive enough for PS. In our case, despite a negative perchlorate test, the patient was diagnosed by mutational analysis and received total thyroidectomy to relieve respiratory distress caused by thyroid enlargement. This is the first report of a mutation detected in the thyroid tissue and clearly shows that the mutation caused histopathologic change in that gland.

Differential diagnosis between Pendred and pseudo-Pendred syndromes: clinical, radiologic, and molecular studies

The disease gene for Pendred syndrome has been recently characterized and named PDS. It codes for a transmembrane protein called pendrin, which is highly expressed at the apical surface of the thyroid cell and functions as a transporter of chloride and iodide. Pendrin is also expressed at the inner ear level, where it appears to be involved in the maintenance of the endolymph homeostasis in the membranous labyrinth, and in the kidney, where it mediates chloride-formate exchange and bicarbonate secretion. Mutations in the PDS gene and the consequent impaired function of pendrin leads to the classic phenotype of Pendred syndrome, i.e. dyshormonogenic goiter and congenital sensorineural hearing loss. In the present study, we performed a detailed clinical, radiologic, and molecular analysis of six families presenting with clinical diagnosis of Pendred syndrome. In two families a homozygous pattern for PDS mutations was found, whereas the affected members of the other four families were compound heterozygotes. One family did not harbor PDS mutations. Among the four novel mutations described, one is a transversion in exon 2 (84C>A), leading to the substitution S28R. Two other novel mutations lie in exon 4 (398T>A) and in exon 16 (1790T>C), leading to the substitutions S133T and L597S, respectively. The fourth novel mutation (1614+1G>A) is located in the first base pair of intron 14, probably affecting the splicing of the PDS gene. Clinically, all patients had goiter with positive perchlorate test, hypothyroidism, and severe or profound sensorineural hearing loss. In all the individuals harboring PDS mutations, but not in the family without PDS mutations, inner ear malformations, such as enlargement of the vestibular aqueduct and of the endolymphatic duct and sac, were documented. The pseudo-Pendred phenotype exhibited by the family without PDS mutations is likely caused by an autoimmune thyroid disease associated with a sensorineural hearing loss of different origin.

Molecular analysis of the Pendred's syndrome gene and magnetic resonance imaging studies of the inner ear are essential for the diagnosis of true Pendred's syndrome

Pendred's syndrome is a combination of congenital sensorineural hearing loss and iodine organification defect leading to a positive perchlorate test and goiter. Although it is the commonest form of syndromic hearing loss, the variable clinical presentation contributes to the difficulty in securing a diagnosis. The identification of the disease gene (PDS) prompts the need to reevaluate the syndrome to identify possible clues for the diagnosis. To this purpose, in three Italian families presenting with the clinical features of Pendred's syndrome, the molecular analysis was accompanied by full clinical, biochemical, and radiological examination. A correlation between genotype and phenotype was found in the only patient with enlargement of vestibular aqueduct and endolymphatic duct and sac at magnetic resonance imaging. This subject was a compound heterozygote for a deletion in PDS exon 10 (1197delT, FS400) and a novel insertion in exon 19 (2182-2183insG, Y728X). The present study demonstrates for the first time the value of the combination of clinical/radiological and genetic studies in the diagnosis of Pendred's syndrome. The positivity of a perchlorate discharge test and the malformations of membranous labyrinth fit well with the recent achievements on the role of pendrin in thyroid hormonogenesis and the maintenance of endolymph homeostasis.

Concurrence of Pendred syndrome, autoimmune thyroiditis, and simple goiter in one family

Pendred syndrome is the autosomal recessively transmitted association of familial goiter and congenital deafness. There is no specific biochemical marker of this disease, and the diagnosis depends upon the demonstration of the triad of congenital sensorineural hearing loss, goiter, and abnormal perchlorate discharge test. Pendred syndrome is caused by mutations within the putative ion transporter gene (PDS gene), located on chromosome 7q. A wide variation in the clinical presentation of this condition, and its well documented phenotypic overlap with other thyroid disorders (such as Hashimoto's thyroiditis), can lead to diagnostic difficulties. The potential for misdiagnosis increases when these disorders occur coincidentally in the same family. We describe a kindred in which Pendred syndrome, autoimmune thyroiditis, and simple goiter coexisted, to highlight these diagnostic pitfalls and to illustrate the use of mutational analysis in resolving diagnostic confusion.

Clinical and molecular genetics studies in Pendred's syndrome

A large and highly inbred kindred including patients with incomplete and complete forms of Pendred's syndrome was studied. Blood samples were collected from 42 individuals (23 affected and 19 normal), and serum thyroid hormones, TSH, Tg, and anti-TPO autoantibodies were assayed. Thyroid function studies have indicated euthyroidism in all 42 individuals. The affected subjects, however, had significantly elevated serum Tg levels (19.4 +/- 6.8 ng/dL) as compared with normals (9.6 +/- 2.9 ng/dL). Nineteen subjects had clinical and or ultrasonographic evidence of a multinodular goiter. In addition, 13 individuals had impaired hearing with or without goiter. Computer axial tomography scan studies in six patients confirmed the presence of a defective cochlea (Mondini's cochlear defect) in three of these subjects. It has been suggested that thyroperoxidase (TPO) in patients with Pendred's syndrome might be defective for coupling but could be partially effective for iodide organification. We have investigated possible abnormalities in the TPO gene by Southern blot analysis. Genomic DNA was obtained from peripheral blood leukocytes of 40 subjects (22 affected and 18 normal). DNA samples were digested with five restriction enzymes and hybridized with the pM5 probe (831 bp). Polymorphic fragment patterns obtained with three of the five enzymes employed were equally distributed in normal and affected subjects of this kindred. Lod score analysis did not disclose any linkage of TPO gene polymorphisms with the phenotypic characteristics observed in this family. Our findings may be explained in two different ways. First one might have hitherto undetected mutations in the TPO gene, and, second, the pathology may in fact be due to a genetic defect lying elsewhere.

CT-scanning of the cochlea in Pendred's syndrome

The inner ears of 5 adult patients with Pendred's syndrome were investigated using a Siemens SOMATOM DRG. Five normal hearing adults participated in the investigation as a control group. The CT-scanning comprised 10-15 consecutive scans of the cochlea. The CT evaluation was performed using both a special bone setting and a soft tissue setting. The cochlear content was quantitatively evaluated by drawing an irregular region of interest on the bone pictures. The region of interest was then transferred to the soft tissue pictures by means of the standard program of the CT-scanner. The resulting mean values of attenuation expressed in Hounsfield Units were significantly lower in the Pendred cochleas than in the normal cochleas. At the same time a typical Mondini malformed cochlea was demonstrated in all patients with Pendred's syndrome. The lower values of attenuation of the cochlea in Pendred's syndrome reflect the rudimentarily developed infra-cochlear osseous structures in this disease. We conclude that CT-scanning of the cochlea using this procedure is reliable enough to replace the conventional axial-pyramidal tomography when a Mondini cochlea is suspected.

The variable intrafamiliar expressivity in Pendred's syndrome

This study presents clinical, audiological, radiological and biochemical data on 14 individuals representing 6 families, each including at least one patient with the classical Pendred's syndrome. The size of the thyroid, degree of hearing loss and result of the iodine perchlorate discharge test showed great variations, even between affected relatives. Based on these observations a new definition of Pendred's syndrome is proposed. The use of the iodine perchlorate discharge test for detection of healthy carriers among family members is suggested.