The findings of our study indicate that 3.2% of studied CH patients diagnosed in newborn screening and treated early and followed up regularly had sensorineural hearing loss, whereas 0.2% of healthy neonates had this problem. The difference was not significant.
The prevalence of hearing loss in control group was similar to that reported by other studies for the general population [23
], but the prevalence of hearing loss among CH patients was lower than in many other studies in this field [11
], though there were also studies with lower [28
] or similar prevalence [28
]. Chou et al in Taiwan, during 1996-98, have reported that 25% of CH patients aged 3-5 years, who received early treatment, had hearing impairment diagnosed by auditory brainstem evoked potentials [26
]. Rovet et al in their study in Canada have indicated that hearing impairment was presented in up to 20% of children with CH diagnosed by newborn screening and treated early[27
Vanderschueren-Lodeweyckx et al have found that in 45 children with sporadic CH with adequate long-term treatment, 9 (20%) had sensorineural hearing loss, particularly in higher frequencies [11
]. On the other hand, Francois et al in France have reported that none of their 49 CH patients had a sensorineural hearing loss that required prosthesis [28
], and similar to our study, according to the results of Bellman et al, 5% of their CH patients with early treatment had hearing problem [29
Parazzini and colleagues, indicated that there was not any significant difference in the prevalence of hearing impairment among untreated CH patients within the first month after birth and control group [30
]. The differences between mentioned studies may be due to different hearing acuity screening tests, age of hearing examination or genetic factors.
All CH patients with hearing impairment had sensorineural hearing loss, which support the previous reports regarding the histological findings in congenital hypothyroid animals. According to these reports, immature development of the organ of Corti including hair cells and tectorial membrane was observed among cases with hypothyroidism [13
]. Our studied population consists of CH patients younger than 3 years old, so they are younger than most studied patients in mentioned studies (3-16 years old). Moreover, we studied all CH patients preliminary diagnosed and treated during CH screening program. They consisted of both transient and permanent CH patients, as permanency of hypothyroidism can be determined after 3 years follow up [31
]. The lower rate of hearing loss in our study could be explained by proper management of CH regarding the mean age in which treatment began or administration of appropriate levothyroxine dosage [31
]. However, it seems that for more accurate conclusion this study should be continued with longer sample size.
There are different reports about hearing impairment and time of starting treatment. In some studies, there was association between hearing loss and delay in initiating therapy [27
], whereas in others, there was no relation found between mentioned factors [28
]. Wasniewska and colleagues have reported that sensorineural hearing loss may be found in CH patients when the substitutive treatment starts many years after its presentation [33
In current study, there was no difference between age of starting treatment in CH patients with and without hearing loss. Our results were in agreement with those of De Laca et al [32
] and Francois et al [28
], but Rovet et al have reported that hearing impaired children, differed from children with normal hearing in age of treatment onset (22 vs. 14 days) [27
Our findings indicate that there was no difference between mean screening T4 and TSH level and hearing impairment, which is in accordance with the results of Rovet et al [27
]. In their study, TSH level at diagnosis was not different in CH patients.
CH neonates with hearing impairment had thyroid dyshormonogenesis according to follow up results, which was in line with the results of Crifo et al. They indicated that dyshormono-genesis was associated with a higher risk of hearing loss among CH patients [16
Unlike Crifo et al Vanderschueren-Ladeweyckx found hearing loss in some patients with an ectopic thyroid gland, too [11
]. However, recent studies reported the relation between hearing impairment and certain gene mutations such as TPO and DUOX2 among CH patients [34
]. So, regarding the fact that the most common etiology of CH in Isfahan was dyshormonogenesis[36
] it seems that further studies in this field including genetic studies should be done.
Considering that there were only 3 CH patients with hearing loss, our results would be more conclusive with larger sample size.