|
Enlargement of the thyroid is common. The Framingham study in the United
States found that 6.4% of females and 1.6% of males had one or more thyroid
nodules (1), while follow-up for 15 years indicated an annual incidence
of 1.4% over that period (1). The Whickham survey, carried out in the northeast
of England, similarly reported that small goitres (those palpable but not
visible) were present in 8.6% of the population, while obvious goitres
(palpable and visible) were present in 6.9%, goitres being four times more
common in females than in males (2). Follow-up of that population for 20
years has recently revealed a reduction in prevalence of thyroid enlargement
with increasing age; amongst women, 70% had no goitre at either survey,
20% had a goitre at first survey but not at follow-up, 6% had goitres at
both surveys and 4% developed goitres (3). While a decline in goitre prevalence
with age has been reported before (1), such surveys highlight the marked
frequency of thyroid enlargement in the population as a whole. The presence
of nodular thyroid disease may be even more common than such epidemiological
studies suggest in that postmortem studies have shown that up to 50% of
the population have either single or multiple nodules in the thyroid, many
of which are very small (4). Studies employing high resolution ultrasound
scans have similarly revealed discrete nodules in up to 50% of those beyond
the fifth decade of life (5).
Aetiological factors determining the development of either single or multiple thyroid nodules are complex. TSH has been widely implicated in the pathogenesis of nodular thyroid disease in view of its role as a potent stimulator of thyroid growth. Growth stimulating immunoglobulins have also been implicated (6), suggesting an underlying autoimmune disorder, while worldwide, iodine deficiency remains a crucial factor in goitre development (7). Previous exposure to ionizing radiation is also a well documented risk factor for the development of both benign and malignant thyroid nodules (8). In contrast to the marked prevalence of thyroid nodules outlined above, thyroid cancer is rare, accounting for less than 0.5% of all new malignancies in England and Wales and less than 0.5% of all cancer deaths (9). Thyroid cancer is reported to have been found in only between 6 and 14% of single thyroid nodules selected for surgery on clinical grounds (10), while cancer risk is even lower in multinodular goitre (11). The picture is, however, complicated by postmortem studies which describe the presence of occult cancer in up to 5% of thyroid glands normal to palpation (4). Most occult cancers are small, being only several millimetres in diameter, and the substantial prevalence of occult cancer in those who have died from unrelated disease suggests that this is an entity without clinical significance. The overwhelming challenge to the clinician is therefore to identify the small number of patients presenting with significant neoplastic disease of the thyroid from the majority without, thus sparing up to 90% of patients unnecessary surgery. |
