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The most common thyroid disease in the community is 'simple'
diffuse goitre. Clinical assessment of goitre size is rather
crude and there is considerable observer variation when using WHO
criteria. Most studies define a thyroid that is visible as well
as palpable as a goitre (WHO grade 1 or above). Considerable
regional variations exist even in non-endemic goitre areas. In
cross-sectional surveys the prevalence of goitre declines with
age. The highest prevalence occurs in pre-menopausal women
although nodular goitres are more common in the elderly.
Prevalence of thyroid nodules is heavily dependant on the
modality of screening and the population being studied(5).
Individuals who received head and neck irradiation at a young age
have a higher than average rate of thyroid nodules and an
increased risk of thyroid cancer. The data on thyroid cancer need
to be seen particularly in relation to the very high frequency of
benign nodular lesions. The prevalence of thyroid nodules by
palpation is between 5-50 per 1000 depending on the survey. Not
surprisingly the prevalence of nodules detected by ultrasound is
much higher than the frequency detected by palpation and is
approximately 300 or more per 1000 and as expected is much higher
in women than in men and increases with age. Palpation alone
seldom enables anyone to distinguish benign from malignant
nodules. Imaging does not allow discrimination between benign and
malignant nodules either.
Imaging may be useful in discriminating between cystic and solid lesions but ultimately fine needle aspiration or biopsy is going to be needed to determine underlying pathology. It would clearly be inappropriate to investigate 30%-40% of the female population on the basis of ultra sound screening. Most people will live a normal life span and die with their thyroid nodules rather than because of them. The fear of thyroid cancer nevertheless remains in the mind of anybody who discovers a thyroid lump and it is of course proper to investigate such individuals by fine needle aspiration but the expectation should be that the vast majority will indeed be benign. The prevalence of thyroid cancer even in patients with thyroid nodules who get as far as surgery will depend on the selection processes used. The use of fine needle aspiration to exclude malignancy in the majority of cases will leave about 20% going to surgery because of equivocal biopsy results. The better the selection criteria the higher the proportion of malignancy in those going to operation. It is important, however, to acknowledge that thyroid cancer accounts for between 1%-2% of all cancers in most populations and the annual incidence of thyroid cancer is between 1-10/100,000 population. The peak rate tends to occur at a younger age in women than in men. The mortality rate from thyroid cancer remains very low. |
