Some patients are clinically euthyroid despite high thyroid hormones levels associated with detectable but not elevated serum TSH. These patients are considered as being resistant to thyroid hormones. The resistance may be severe or partial and in most cases seems to be autosomal dominant; it affects some tissues more than others, thus giving rise to a variety of clinical symptoms. In a few patients with high TSH levels extensive studies have failed to provide evidence of pituitary tumour of resistance to thyroid hormones; the cause of TSH hypersecretion in such cases remains uncertain. Several factors (non specific serum proteins, cross-reactivity circulating antibodies) may result in falsely high T3, T4 and TSH values on radioimmune assays and must be carefully investigated in the presence of unexpectedly high serum TSH levels.