Ultrasound image of the thyroid gland in obese children.

Obesity as a disease of affluence also affects younger children. Numerous observations suggest a link between excessive body weight and thyroid function disorders. Subclinical hypothyroidism has been diagnosed increasingly frequently in patients with obesity. A growing number of papers also point to morphological changes of the thyroid gland in the ultrasound examination in obese children. These reports mainly concern changes in echogenicity. The present paper discusses the most important aspects of this topic on the basis of the literature as well as containing a brief analysis based on own experiences.

Introduction

Simple obesity is currently one of the most common metabolic diseases and its incidence has been increasing every year. This is mainly due to changes in lifestyle and dietary habits of contemporary society as well as the development of inappropriate nutrition and physical activity patterns already in early childhood. Obesity increases the risk of hypertension, dyslipidemia and abnormal glucose tolerance, which results in an increased risk of heart and metabolic diseases(.

Obesity as a disease of affluence affects the youngest patients increasingly frequently. Over the last decades a gradual increase in the prevalence of excessive body weight and obesity has been observed around the world. This is true not only for the adult population, but for children as well. The estimated number of overweight or obese pediatric patients exceeds 110 million today. It is thought that if this trend is not stopped, the present generation of children, despite the considerable progress in medical sciences, will probably be the first generation to live shorter than the parents.

The additional tests for children with simple obesity most commonly ordered by primary care physicians include the assessment of thyroid function conducted in order to exclude hormonal imbalances that cause obesity. Obese individuals relatively often have elevated levels of the thyroid-stimulating hormone (TSH) in the blood serum, called hyperthyrotropinemia, which has a tendency to rise with age. The substance which is responsible for this is leptin, which regulates body weight and hunger, influencing the level of TSH in the blood serum by modifying its synthesis in the pituitary gland(. Another mechanism accounting for the elevated levels of this hormone in obese patients consists in the possible resistance to thyroid hormones at the level of the pituitary gland and disruption of the negative feedback phenomenon. As a result, the TSH serum level does not decrease despite a normal level of thyroxine. The TSH concentration is observed to decrease following the reduction of body weight to a normal level(.

An attempt at broadening the diagnostic investigation by conducting an imaging examination of the thyroid gland, in addition to tests of free thyroxine, triiodothyronine and antibodies against thyroid peroxidase and thyroglobulin seems to be an obvious course of action. It is consistent with the current sonography standards. According to them one of the indications for the ultrasound examination of the thyroid gland is a suspected disease based on abnormal laboratory test results, including hyperthyroidism, hypothyroidism and thyroiditis(. Correct interpretation of a thyroid ultrasound examination in an obese child may become one of the fundamental elements affecting further course of action or therapeutic decisions, especially with borderline values of TSH or thyroid hormones. Therefore, it is necessary to analyze all elements of the examination carefully.

Lesions in the thyroid gland which may be associated with obesity have been reported in medical literature for some time. The most important ones include increased volume of the gland, changed echogenicity and the presence of focal lesions.

Thyroid volume

In a Dutch study it was demonstrated that adult obese patients had a larger thyroid gland volume in comparison with a group of individuals with a normal body weight. Thyroid volume correlated with lean body mass and TSH and not with body fat mass(. In another study it was observed that the thyroid gland volume in healthy euthyroid women may change with the changes in body mass. A larger thyroid volume, although still within the reference range, was demonstrated in obese women. Correlation between this parameter and body mass, body mass index (BMI) and waist circumference was found. The thyroid volume decreased in patients who after a dietary intervention reduced their body weight by at least 10% within 6 months(.

Slightly different data have been collected in pediatric populations. Based on the analysis of a group consisting of 280 obese children Stichel et al. found that the volume of the thyroid gland in the studied patients fell within the normal range. The thyroid gland volume in relation to the surface area of the subjects’ bodies also did not differ from the values found in healthy children from the Berlin population(.

Focal lesions

Over the last few years a possible link between thyroid cancer and obesity has been reported in medical literature. In large screening studies on adults in Seoul a significantly higher rate of concurrent obesity and thyroid cancer in women was observed. However, such a relationship was not found to occur in men. A link between the prevalence of thyroid cancer and age, insulin levels or TSH was excluded(. No reports of such studies in children exist. One needs to bear in mind, however, that the incidence of thyroid cancer in the youngest patients, however low, seems to be rising. Certainly during the coming years numerous studies will be conducted in this direction.

Change in echogenicity

Over the last few years papers appeared which suggested that in obese children ultrasound examinations of the thyroid gland often demonstrate changes similar to those observed in the autoimmune Hashimoto thyroiditis(. However, no antibodies against the thyroid gland were found in the blood of these patients and fine-needle biopsy did not reveal lymphocyte infiltration, which excludes the presence of Hashimoto thyroiditis. The authors put forward the hypothesis that the reported lesions in the thyroid gland are linked to a chronic inflammatory process associated with obesity. The adipose tissue, as an endocrine organ, secretes inflammatory cytokines such as the tumor necrosis factor α (TNF-α) and interleukines 1 and 6 (IL-1 and IL-6), which may induce a low-intensity inflammatory process in obese patients(. Cytokines are probably responsible for the dilation and increase in the permeability of the thyroid blood vessels, which may result in an effusion into the thyroid parenchyma that forms a sonography image typical of the Hashimoto's disease. Ultrasound examination in children also revealed the return of the thyroid structure to the normal state following weight loss and lifestyle change(. This phenomenon is probably associated with the reduction of the intensity of the inflammatory process, which may cause a regression of lesions in the thyroid parenchyma as a result of weight loss(. Similar changes in adult patients are reported by Rotondi(.

One should bear in mind that the English term „echogenicity” covers two Polish terms: „echogeniczność” („echogenicity”) and „echostruktura” („echostructure”). Autoimmune inflammation is usually considered to be characterized by both decreased echogenicity and abnormal echostructure. In addition, in the English-language medical literature often the slightest deviation from the norm is believed to be sufficient to identify the pattern suggesting Hashimoto's disease, which leads to numerous overinterpretations. It is worth taking into account the fact that these are relatively general concepts and that the image may not necessarily correspond to the extended criteria for the diagnosis of autoimmune thyroiditis in children described in medical literature. Other features which are helpful for the diagnosis of Hashimoto's disease include increased parenchymal flow and the presence of lymph nodes below the lower poles of the gland. It is only the coexistence of these features that provides grounds for an ultrasound diagnosis of autoimmune thyroiditis. One should bear in mind that it is not tantamount to Hashimoto's disease; it is one of the types of autoimmune thyroiditis and a differential diagnosis is not possible without clinical data.

It is also worth remembering that Hashimoto thyroiditis is more prevalent in obese children than in the general population. Obese children are found to have a higher prevalence of elevated levels of antibodies against the thyroid gland than the general population, especially in the case of the presence of elevated TSH levels. This is probably associated with an increased presentation of the antigen caused by a stronger stimulation of the thyroid(.

The authors of the present paper were interested in this subject and for this reason they started their own research into the matter, the preliminary results of which were presented in the form of a poster during two congresses: European Society for Paediatric Endocrinology and Euroson meetings(. This research indicates that the ultrasound image of the thyroid gland in obese children is clearly changed; however, it is not typical of an autoimmune inflammation. Two obese pediatric groups were analyzed: one formed in 2005–2007 (58 children) and the second one formed in 2011–2015 (109 patients). In both groups a characteristic sign of decreased echogenicity of the gland was present (down to 61%), especially in the peripheral areas of the lobes. Echostructure disorders are less frequent (33%), both in combination with changed echogenicity and alone (9.2%). Single cases of Hashimoto's disease were diagnosed. In the vast majority of the remaining patients (93%) neither increased flow nor the presence of lymph nodes in the area of the lower poles were found. A very distinct feature is the abovementioned peripheral distribution of decreased echogenicity, which was found in both groups in 47% and 61% of patients, respectively. It is usually arranged in such a way that the central and medial parts of both lobes have a normal echogenicity in almost every case, while in the remaining parts, especially dorsally, echogenicity is decreased, sometimes heterogeneously. This decrease might partly be explained by the phenomenon of tissue attenuation; however, it does not account for the changes in echogenicity in the ventral and lateral fragments of the lobes. A possible cause may be the one suggested by Radetti – a low-intensity inflammatory process or an adipocyte infiltration. Such an image, however, is substantially different from the image observed in patients with autoimmune thyroiditis. These differences are visually presented in Figure 1 and listed in Table 1.

Fig. 1

Ultrasound image of the thyroid gland: A. Normal B. Typical for Hashimoto's disease C. Typical for obesity

Tab. 1

Basic differences between the thyroid image in healthy children, obese children and children with autoimmune thyroiditis

	Healthy thyroid gland	Autoimmune inflammation	Thyroid in obesity
Decreased echogenicity	-	+ + +	+
Echostructure disorders	-	+ + +	+/-
Increased flow	-	+	-
Lymph nodes	-	+	-

In summary, the interpretation of this phenomenon is inconclusive and should be based on a very detailed ultrasound examination of the thyroid gland. Therefore, when assessing the echogenicity of the thyroid gland in obese children, one should remain very cautious about the interpretation of the changes and in case of doubt the diagnosis should not be exclusively based on an abnormal ultrasound image without proving the presence of antibodies against the thyroid gland.