Subclinical hypothyroidism and hyperthyroidism. I. Prevalence and clinical relevance.

Subclinical hypothyroidism has a prevalence of approx. 6% in the general population; it is more common in females and in the elderly. The incidence of progression to overt hypothyroidism is 5-15% per year; women with positive thyroid antibodies are especially at risk. The biological significance appears to be small; there may be an association with depression. Subclinical hypothyroidism does not cause significant hypercholesterolaemia. Thyroxine treatment results in improvement of symptoms in 25-30%. Subclinical hyperthyroidism has a prevalence of approx. 1%; it is also more common in older age groups, but its female preponderance is less marked. The incidence of progression to overt thyrotoxicosis is approx. 5% per year; subjects with autonomous thyroid adenoma or nodular goitre are especially at risk. The biological significance appears to be small. Bone density is slightly reduced in cortical bone (radius and femoral neck) but not in trabecular bone (lumbar spine). There might be an association with atrial fibrillation, which is possibly more likely to convert to stable sinus rhythm after antithyroid treatment. In view of the high prevalence of subclinical hypothyroidism and hyperthyroidism one might consider screening programs in the general population, which are feasible by the availability of an appropriate screening test (the sensitive TSH assay) and effective treatment. Such screening programs, however, are not justified at the present time because (a) the associated burden of disease is small and (b) it has not been proven beyond doubt that early diagnosis and treatment in the asymptomatic phase improves clinical outcome. A high degree of suspicion of thyroid function disorders is, however, warranted, especially in females over 40 years presenting with non-specific complaints.