Although thyroid dysgenesis is known to be the most common cause of congenital hypothyroidism, it has been shown in recent studies that the incidence of dyshormonogenesis has increased. While dyshormonogenesis is involved in the etiology of 15% in newborn screening, this rate has been shown to be 30–40% with the latest newborn screening strategies (12–14). The majority of CH were caused by TD and < 20% of patients showed strong genetic predisposition (14).
In the present study, the frequency of dysgenesis was 19.2%. The gene causing dysgenesis was identified in four patients (4/10, 40%). One patient had goiter despite the NKX2-1 mutation (patient 17). One patient also had a GNAS gene mutation (patient 18). The prevalence of dyshormonogenesis was 40.3%, which supports the recent studies (15, 16, 17).
Heterozygous loss-of-function mutations in NKX2-1 gene have been reported to cause a complex phenotype called brain-lung-thyroid syndrome (18). In a study examining 183 patients with congenital hypothyroidism, it was reported that four patients had NKX2-1 mutation. Two patients had lung problems, and no respiratory or neurological problems were reported in two patients. Thyroid gland of one patient was normal. It has been emphasized that it can cause hypothyroidism in different phenotypes (19). In our patient, there was goiter despite the NKX2-1 mutation (patient 17). There was no patient with goiter reported in the literature.
Recent studies have shown that the frequency of gene defects associated with CH is substantially higher than previously estimated, and ranges from 33,0% to 61,5%. However, these studies were limited by either the number of genes selected for analysis (15, 16, 17) or the number of the patients included in the study (15, 20, 21).
A total of 32 target genes were investigated for all patients in the present study. The number of genes studied was quite high compared to previous studies. This led to the identification of more variants. The identification frequency of mutations was 55% (29/52). 33 different variants were identified in 29 patients.
Studies on thyroid dyshormonogenesis genes have shown a high rate of mutations. On the other hand, the mutation rate was found to be 70% in a study, and most of them were heterozygous mutations (22). In another study, the frequency of gene mutations causing dyshormonogenesis was 84.8% (23). In the present study, the identification rate of dyshormonogenesis genes was 70%. Heterozygous mutation frequency was very high. The results of our study was in accordance with other studies.
Most of the mutations in recent studies were VUS. It has been emphasized in studies that the pathogenicity of genes specified as VUS should be better evaluated (24).
The frequency of VUS was also high in the present study. It is considered that the presence of mutations in more than one gene in the same patient causes clinical symptoms even though the mutation is VUS. Patients with variants of VUS should be better evaluated clinically.
The most frequent mutations in studies were in the DUOX2 gene (25). There are also studies in which the TPO gene variant is rarely found with a high frequency. In a study 12 candidate genes were investigated. The frequency of pathogenicity was 37.9% (23). In a study investigating only DUOX2 and DUOXA2 genes, the mutation frequency was 50% (26). In the present study, the frequency of DUOX2 gene was high. The mutation frequency with proven pathogenicity was 26.4%. Consistent with the literature, most of the patients did not have goiter. TG and TPO gene changes were also found to be the 2nd in frequency. It was thought that the reason for not developing goiter might be the antiguatrogen effect of early levotroxine treatment (26).
One of the 2 siblings with SLC5A5 mutation had goiter, while the other had hypoplasic thyroid tissue. The cause of hypoplasia may be due to drug use, or it was thought to be a concomitant dysgenesis gene. Only one study performed in mice demonstrated a multigenic origin of CH with TD (27). Goiter formation and the size of thyroid volume are affected by factors such as overstimulation by TSH, duration of exposure to elevated TSH, and iodide exposure. It has also been suggested that hormone replacement therapy in the early period may prevent the formation of goiter (28). The facts that one of the two siblings with the same mutation had goiter and the other had hypoplasic thyroid tissue is one of the interesting results of our study.
It was thought that the presence of multiple mutations could be demonstrated by future studies investigating multiple genes in the same patient via NGS platforms.
There are some limitations to the present study. One of the limitations is that parental genes couldn’t be studied. The accurate effect of some variants on the phenotype couldn’t be determined as some patients were still under follow-up, and the distinction couldn’t be made between transient and permanent hypothyroidism in all patients. Iodine level could not be measured in the study center; therefore, we could not distinguish between transient and permanent hypothyroidism in the early period. Another limitation was that MLPA (multiplex ligation-dependent probe amplification) could not be performed in patients with heterozygous mutations due to the lack of suitable probes. Moreover, possible intronic regions were not examined as only the exon regions of the genes were screened.
In congenital hypothyroidism, the treatment is discontinued at 3 years of age with the diagnosis of transient hypothyroidism in patients with unproven dysgenesis. However, in patients with dyshormonogenesis, which is known to cause iodine deficiency, iodine overload, and transient hypothyroidism, the treatment can be discontinued much earlier. In a study, the frequency of transient CH was found to be 52%, and it was shown that the treatment could be discontinued before the age of 3 (29).
In the present study, the treatment of two patients was discontinued at an early stage based on the genetic result. One of these patients was 7 years old and was receiving low-dose therapy (patient 12, DUOX2). The treatment was discontinued when the mutation causing transient hypothyroidism was detected. When the second patient was 1 year old, he received low-dose treatment, and his treatment was discontinued early based on his genetic results (patient 8, DUOX2).
It is considered that in this period when genetic studies has become widespread and the etiology can be clarified by simultaneously studying many genes with NGS, the investigation of CH genes can prevent unnecessary treatment extension.