Introduction
The importance of thyroid disorders is being increasingly recognized by general practitioners and physicians alike. A few points are noteworthy in this context. Firstly, thyroid disorders are commoner than what we might expect. Diagnosed cases usually form a small fraction of the total patient load in the community. Secondly, thyroid disorders have protean presentations. By their very nature, the symptoms could involve more than one body system. It is important to have a high degree of suspicion for thyroid disorder diagnosis.
"The eyes do not see what the mind does not know" is often true for thyroid disorders! Thirdly, thyroid disorders can be confidently diagnosed with the availability of ultra sensitive, third-generation TSH assays and completely treated with reliable drug preparations in different strengths to tailor precisely to individual requirements. Treating a patient with a thyroid disorder is usually a dramatic and gratifying experience.
Thyroid Disorder Burden In The Community
Hypothyroidism and hyperthyroidism account for considerable morbidity, rarely, mortality can occur due to thyrotoxic crisis (thyroid storm) and myxedema coma. The Whickham survey done in the Whickham district of north-eastern England near Newcastle-upon-Tyne is a landmark epidemiological study. Between 1972-1974, 2779 randomly selected adult subjects (age 18 years and above) residing in this mixed rural and urban area were surveyed to determine the prevalence of hypothyroidism, hyperthyroidism, autoimmune thyroiditis and goitre in the community.
The sample included 1285 men (46%) and 1494 women (56%) with a mean age of 47 years. The age, sex and social class of the participants were similar to that of the population of Great Britain as a whole which in turn is probably a reflection of the population of northern Europe. Approximately 20 years after the original Whickham survey, a follow up survey was conducted from March 1992 to August 1993 to evaluate the incidence and natural history of thyroid disease. Remarkably, the follow up survey provided information on 2625 (95%) participants of the original survey done 20 years ago! The salient results of this monumental survey are summarized below.
- The prevalence of elevated TSH levels, overt hypothyroidism / hyperthyroidism, goitre, elevated antithyroid antibodies are much higher in women compared to men
- The prevalence of overt hypothyroidism was 18 per 1000 in women of which 3 per 1000 were previously unsuspected. The number of women with elevated TSH levels (> 6 mU/L) was substantial viz. 7.5%. In contrast, the prevalence of hypothyroidism in men was only 1 per 1000; none of the men studied had unsuspected hypothyroidism. The number of men with elevated TSH levels was 2.8%. Thus, TSH screening in the community would pick up mostly persons (particularly women) with subclinical hypothyroidism. Interestingly, TSH levels showed a progressive increase with time in women but not in men, the increase was almost entirely in those with raised antithyroid antibodies. The risk for hypothyroidism is increased substantially when the serum TSH level is elevated and serum antithyroid antibodies are positive, either alone or together. As the serum TSH rises above 2 mU/L, there is a sharp increase in the probability of hypothyroidism independent of age or antibody status.
- The prevalence of overt hyperthyroidism was 28 per 1000 in women (unsuspected in 5 per 1000) and 2 per 1000 in men ( unsuspected in none). Serum TSH levels were low (<0.5 mU/L) in 10% women, none of the men showed a suppressed TSH. One must realize that the TSH assays employed were not sensitive enough. Subsequent studies with more sensitive assays have estimated the prevalence of subclinical hyperthyroidism to be 1%.
- Goitres were observed in 15.5% of the participants, including 8.6% with small goitres and 6.9% with obvious large goitres. There was no difference in the urinary iodine outputs of those with or without goitres. Large goitres showed a 13 to 1 female: male ratio. There was no association between positive antithyroid antibody titres and goitre in men and only a small association in women. Goitres are common in women, uncommon in men and tend to disappear with time.
- The incidence of spontaneous hypothyroidism and hyperthyroidism in women are low at 3.5 and 0.8 cases per 1000 subjects per year respectively. The incidence of spontaneous hypothyroidism in men is 0.6 cases per 1000 per year. None of the male survivors surveyed over 20 years had a new case of hyperthyroidism.
Other large epidemiological surveys carried out in different patient populations in Sweden, Japan and the United States have more or less shown similar results as the Whickham survey. Thus, low TSH levels are common in the community, undetectable TSH levels are less common and subclinical hyperthyroidism is uncommon with a prevalence of less than 1%. In contrast, subclinical hypothyroidism is a common disorder present in approximately 5% of the adult population and 10-20% of elderly women. Initial serum TSH levels and antithyroid antibody titre are both predictive of minor and major degrees of subsequent thyroid gland failure (about 5% per year when both are present together).
