The development of nodular goitre is influenced by extrinsic factors interacting with intrinsic factors and constitutional parameters of gender and age.

The most important trigger for nodular (and diffuse) goitre is iodine deficiency. There is a direct correlation between goitre prevalence and iodine deficiency and vice versa between correction of iodine deficiency and regression of goitre incidence. This indicates that iodine supply or intake has to be adjusted adequately to reduce the burden of thyroid goitre development in a population.

Iodine deficiency was common in Germany until the early 1990s. Introduction of iodized salt into food industries resulted in a marked improvement in nutritional iodine supply as reflected by an increased urinary iodine excretion (median 72 µg iodine/ L urine in 1994 to 125 µg iodine/ L urine in 2003). Also, the percentage of private households covered with sufficient iodized salt increased from less than 10% to 66% between 1990 and 2002.

In 1994 the prevalence of diffuse goitre was 21% in the age group of 18– 30 years and 33% in the age group of 46– 65 years. In the Papillon study of 2002 investigating 96 000 German employees an impressive reduction in goitre frequency to 6% in the group of 18– 30- year- olds and to 26% in the group of 46– 65- year- old participants was found. The epidemiological study SHIP- 0, has underscored that with a decrease in overall- goitre prevalence due to improved iodine supply, thyroid nodules now tend to occur in normal size rather than enlarged thyroid glands. This may be explained by the thyroid inherent disposition to develop focal hyperplasia as discussed next.

Recently, two German studies in children (KiGGS) and adults (DEGS) showed results that the iodine status may not be sufficient. In both surveys about 25– 30% of the individuals were below the estimated average requirement for iodine. This was also supported by comparing results from the SHIP- 0 (1997– 2001) and SHIP- TREND (2008– 2012) study in an adult population of Northeast Germany. The median urinary excretion levels significantly decreased from 123µg/ L to 112µg/ L between 2000 and 2010. These data clearly indicated that informative advertising of iodine prophylaxis needs to be intensified to maintain and still to improve iodine status.

Various other goitrogenic factors are known and are relevant to thyroid disease in situations with coexisting iodine deficiency. First, metabolites of various nutrients (e.g. cabbage, cauliflower, and broccoli) may interfere with iodine uptake. Second, industrial pollutants, including resorcinol and phthalic acid, are known to be goitrogenic. Third, deficiencies of selenium, iron, and vitamin A may exacerbate the pathogenic effects of iodine deficiency.

Other risk factors for nodular goitre have been suggested, whereby a possibly distinct impact on the prevalence of thyroid nodules occurring in a normal sized or enlarged thyroid gland is less clear. Smoking has been proposed as a risk factor for goitre and nodules were also found with higher prevalence in goitres of smokers compared with non- smokers. The impact of smoking on thyroid disease is most likely due to increased thiocyanate levels in smokers exerting a competitive inhibitory effect on iodide uptake. In line with this, the association is more pronounced in areas with iodine deficiency.

Radiation is another environmental risk factor not only for thyroid malignancy but also for benign nodular thyroid disease. An increased prevalence of thyroid nodules disease has been associated with exposure to radio nuclear fallouts and therapeutic external radiation.

Nodular thyroid disease and goitre are more frequent (2.5- to 7- fold) in women and it seems that this prevalence appears after puberty. The determined gender difference is more pronounced in areas of long- standing iodine deficiency compared with regions of iodine sufficiency. However, the precise reasons for this observation remain to be clarified. A growth promoting effect of oestrogens has been described in vitro and oestradiol has been suggested to amplify growth factor- dependent signalling in normal thyroid cells and thyroid tumours. However, pregnancy- related thyroid enlargement appears to be mostly related to iodine deficiency and in one German study increased MNG prevalence with parity was only observed in women who had not taken iodine supplementation during an earlier pregnancy.

Several studies suggest that thyroid volume is also significantly correlated with body mass index. In agreement with this, a recent study has shown that in obese women, weight loss of more than 10% may result in a significant decrease in thyroid volume. Lastly, because of the cumulative impact of external risk factors on the thyroid gland, the prevalence of thyroid nodular disease increases with age.

For example, a study (DanThyr) from a borderline iodine- deficient area in Denmark demonstrated the relationship of nodular goitre development in ageing women. The survey of 4242 women showed a goitre incidence of 18% with participants aged 20– 39 years exhibiting higher prevalence for solitary thyroid nodules than for MNG. The prevalence of these two entities is similar in the fourth decade of life but shifts to MNG after 50 years of age.

اخر الاخبار

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مواضيع ذات صلة

Pathogenesis of Non-Toxic Goitre: Genetic Disposition

Management of Subclinical Hypothyroidism - efficacy of Levothyroxine Treatment

Long- Term Complications of Subclinical Hypothyroidism

Myxoedema Coma: Thyroid Hormone Therapy

Myxoedema Coma: Hyponatraemia

Diagnosis of Myxoedema Coma

Pathophysiology of Unstable Hemoglobin Disorders

Central Hypothyroidism

Severe Iodine Deficiency and natural Goitrogens

Iatrogenic Hypothyroidism

Pathobiology of Sickle Cell Disease: The Basis of Phenotypic Diversity

The Unique Systems Biology of Sickle Cell Anemia