In the present study, comparison of the findings of ultrasonography with scintigraphy, demonstrated that this imaging technique is useful for early diagnosis of different etiologies of CH during screening program. The findings of this study, likewise other studies, showed that ultrasonography could reflect the anatomical status of the thyroid gland.14
However, ultrasonography had some weaknesses in diagnosing ectopic thyroid gland.18
In this study, only 33% of ectopic thyroid glands were diagnosed by ultrasonography. In another similar study which was performed among 54 CH patients, only 5 out of 26 cases with ectopic thyroid gland were diagnosed by ultrasonography.19
In a similar recent study in Korea, discordant cases of CH according to the comparison of findings of ultrasonography and scintigraphy were investigated; in 6/300 patients, ultrasonography was not able to detect the ectopic thyroid gland and its sensitivity in this field was 78%, which was higher than ours. The specificity of ultrasonography in this field was 100%, which was similar to our results.20
Another weakness of this imaging tool is its limited value for the evaluation of thyroid gland function, whereas scintigraphy reflects the functional status of the gland.14
Therefore, it may identify an anatomically normal but nonfunctional gland.11,14
In the current study, 63/102 cases were reported to have normal thyroid gland by ultrasonography and this proportion was 57/102 for scintigraphy, but according to final diagnosis, 53 cases were normal. In 10 cases of agenesis, ultrasonography reported normal gland, which consequently decreased the specificity of ultrasonography to 92 %.
Change et al. reported 42/300 subjects with normal thyroid gland by ultrasonography and this proportion was 55/300 for scintigraphy.20
In the study of De Bruyn et al., in 54 hypothyroid neonates, 4 nonfunctional thyroid glands were reported as having normal thyroid gland by ultrasonography.
These findings could be due to maternal suppressing TSH receptor blocking antibodies, transient elevated thyrotropin, hypopituitarism and unknown causes.19,20
So, it seems that mentioned factors should be evaluated in these cases in future studies.
Several studies have investigated the usefulness of ultrasonography in determining different etiologies of CH and they have reported controversial results in this field. Some of them concluded that ultrasonography has limited value in diagnosing the etiologies of the disease. Muir et al. in their comparative study between ultrasonography and scintigraphy among 54 congenially hypothyroid neonates have reported that none of 13 patients with thyroid ectopia was detected with ultrasonography and 4 cases of agenesia had a normal gland according to ultrasonographic results. Therefore, they concluded that ultrasonography could not be considered as the alternative method to scintigraphy to define the causes of CH.12
Other authors confirmed the usefulness of ultrasonography specially for the initial evaluation of CH and they recommended that scintigraphy should be used as a complementary imaging tool where ultrasonography is not capable of identifying accurately the etiology of CH. But, the indication of using scintigraphy was different in the mentioned studies.
Bubutershvili et al. in their study among 66 CH patients, in 2003 in France, have reported that in 12 cases for which no iodine uptake was demonstrated by scintigraphy, ultrasonography showed normally located thyroid tissue in 2 patients and the sensitivity of ultrasonography in identifying ectopic tissue was 21%. They concluded that ultrasonography may reveal additional findings with regard to those obtained by scintigraphy such as information regarding the anatomy and morphology of the thyroid gland and scintigraphy is required in patients with no visibility of the thyroid gland in its normal location or those with goiter.21
Takashima et al. have reported that ultrasonography showed 75% sensitivity for detecting thyroid ectopia. According to their recommendation, scintigraphy is necessary for patients with absent gland in normal location on ultrasonography to confirm ectopia and to differentiate between ectopia and true aplasia. According to their final report, careful ultrasonography of the neck in association with biochemical laboratory data is enough in more than 54% of CH patients and the indication of scintigraphy was similar to that recommended by Bubutershvili et al.14
Although Kreisner et al. in their study among 89 cases of CH have the same conclusion about the utility of ultrasonography, their recommendation about the indication of scintigraphy assessment was different. They concluded that scintigraphy should be used to discriminate between different types of dysgenesis identified by ultrasonography.11
Recently, Perry et al. determined the strengths and weaknesses of two mentioned imaging techniques in 40 primarily diagnosed CH neonates and they concluded that the scintigraphy had superiority in diagnosing thyroid ectopia whereas ultrasonography had ability to detect tissue that was not visualized on scintigraphy and to show abnormalities of thyroid volume and morphology. So, they concluded that combined ultrasound and isotope scanning is more informative in the diagnosis of CH etiology.13
Change et al. in their recent similar study in Korea have reported the same conclusion.20
The limitation of the current study was that we did not evaluate inter- and intra-observer CV.
In sum, given mentioned reports and the results of our study, though ultrasonography had failed to diagnose 67% of ectopic cases and nonfunctioning thyroid glands, it had the ability to determine the anatomy of thyroid gland. So, it seems that the combined imaging methods would be more preferable in this field. But, considering our experience during CH screening program which indicated that most parents refused to do scintigraphy because of its limitations and their fear of radiation, our recommendation is to consider further scintigraphic evaluation of ectopic cases failed to be diagnosed by ultrasonography and were misinterpreted as agenesia or hypoplasia of the thyroid gland; because otherwise they will be finally classified as thyroid dysgenesis and this could explain the low rate of CH due to ectopia in our studied population in contract to others16
. On the other hand, the superiority of ultrasonography in assessment of gland morphology can introduce it as a relatively appropriate imaging tool for diagnosing CH etiologies especially in the initial phase of CH screening. In cases with normal thyroid gland reported by ultrasonography, in order to determine the etiology of CH (transient or permanent CH with dyshormonogenesis or non-functioning thyroid gland), first we could consider the screening tests of TSH and T4 levels. However, Iranpour et al. in their study in the same population in Isfahan have reported that neonates with thyroid agenesia had significantly higher serum TSH value during screening.22
Thyroid scintigraphy was able to diagnose these cases that both ultrasonographic and biochemical findings were not able to correctly determine the etiology of CH for them. In addition, thyroid scintigraphy should be assessed to discriminate between agenesia and ectopia when CH patients reevaluated at 2-3 years old to determine the permanency of CH.