Endocrine Abstracts

Subclinical hyperthyroidism is characterised by low serum TSH with normal free T₄ and free T₃. Subclinical hyperthyroidism may reflect Graves’ disease or toxic nodular hyperthyroidism, and is found in up to 5% of the over 60’s. In addition, about 20% of patients who are taking T₄ therapy have low serum TSH, i.e. biochemical evidence for over-treatment. In those not prescribed T₄ it is important to exclude other causes of low TSH, especially non-thyroidal illness and drug therapies. The potential risks of subclinical hyperthyroidism are cardiovascular and osteoporosis. Effects on cardiac function are well documented, including increased risk of AF and vascular mortality. Bone mineral density may be reduced, especially in post menopausal women, and there is evidence of an effect on fracture risk. There are no studies of intervention with clinical endpoints, although some evidence of improvement in BMD. Guidelines suggest consideration of treatment in the elderly, those with subclinical hyperthyroidism proven to be Graves’ disease or toxic nodular goitre, especially in those with AF or vascular disease.

Subclinical hypothyroidism is high serum TSH with normal free T₄. If a standard cut-off for serum TSH of 4.5 mU/l is adopted, the prevalence is ~10% of the over 60’s and rises with age. Subclinical hypothyroidism is also found in 25% of those taking T₄. Possible associations include hyperlipidaemia (and vascular risk), impaired cognitive function, and impaired well being. Evidence suggests that subclinical hypothyroidism has a finite rate of progression to overt thyroid dysfunction (although rate is variable) and there is weak association with total and LDL cholesterol. Epidemiological studies have revealed conflicting evidence regarding the risk of vascular end points but there may be an association with IHD. In contrast, there is a lack of association of subclinical hypothyroidism with changes in cognitive function, neuropsychiatric symptoms or well being. Most studies of intervention with T₄ show minor effects on lipids and minimal/no effect on cognitive function or symptoms. The main indication for treatment of subclinical hypothyroidism is therefore risk of progression to overt disease. Guidelines suggest consideration of T₄ in those with TSH >10 mU/l, but not in those with lower TSH. An exception to this is pregnancy in which even mild subclinical hypothyroidism should be treated because of a possible association of maternal thyroid dysfunction with effects on neurodevelopment in offspring.