The most common cause of these test results is excessive thyroxine ingestion.
Otherwise the combination of a suppressed TSH and normal free thyroid hormones is usually an asymptomatic disorder termed subclinical hyperthyroidism. In the elderly subclinical hyperthyroidism is often caused by multinodular goitre.
Rare causes of low TSH with normal free thyroid hormones are seen in hospital practice:
Complete suppression of TSH (<0.1 mU/L) merits treatment for hyperthyroidism because of the significant increased morbidity associated with subclinical hyperthyroidism
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