The most common thyroid disease is simple (diffuse) goitre. The clinical grading of thyroid size is subjective and imprecise. The WHO grading system recognizes that an enlarged thyroid gland may be palpably but not visibly enlarged. Interexaminer variation may lead to differences in classifying a goitre as diffuse or multinodular. There is considerable overlap between the five WHO grades based on clinical criteria and thyroid volume estimated by ultrasonography. Ultrasonography has been used in epidemiological studies to assess thyroid size, resulting in much higher estimates of goitre prevalence than in studies in which goitre size was assessed by physical examination.

In cross- sectional surveys, the prevalence of diffuse goitre declines with age, the greatest prevalence is in premenopausal women, and the ratio of women to men is at least 4:1. In the Whickham survey, among the women 26% had a goitre; the frequency ranged from 31% in those aged less than 45 years (mostly diffuse) to 12% in those aged over 75 years (who had a higher proportion of nodular goitre)  (Figure1.). Longitudinal studies confirm the decreasing frequency of diffuse goitre with age. This decline in frequency of diffuse goitres with age is in contrast to the increase in frequency of thyroid nodules and antithyroid antibodies with age. In the Whickham survey, less than 1% of the men but 5% of the women had thyroid nodules detected clinically, and the frequency increased to 9% in women more than age 75. A higher prevalence of nodular goitre is found in areas of iodine deficiency in Europe, such as Italy, Germany, and Denmark. Longitudinal data suggest an annual incidence for nodules of 1/ 1000 and that, once formed, they tend to remain present and benign for a long period of time.

Fig1. Age and sex distribution of thyroid microsomal antibodies (Ab), raised serum TSH greater than 6 mU/ L (↑TSH), visible diffuse and multinodular goitre (G), and nodules (N) in the Whickham survey. Reproduced with permission from Tunbridge WMG, Evered DC, Hall R, Appleton D, Brewis M, Clark F, et al. The spectrum of thyroid disease in the community: the Whickham survey. Clin Endocrinol, 1977; 7: 485. Copyright © 2008, John Wiley and Sons, Blackwell Science.

With the increasing use of sensitive imaging techniques, an increasing proportion of thyroid nodules are detected incidentally. Many nodules are detected because of their size or anterior position in the neck, or the skill of the physician performing the examination but most thyroid nodules will not be clinically recognized. Up to 50% of nodules more than 1 cm detected by ultrasound are undetected by clinical examination. The prevalence of thyroid incidentaloma as an unexpected, asymptomatic thyroid nodule discovered during the investigation of an unrelated condition, is 67% with ultrasonography imaging, 15% with computed tomog raphy or magnetic resonance imaging of the neck, and 1– 2% with f luorodeoxyglucose positron emission tomography.

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