In an unselected series of 239 patients with uncomplicated hyperthyroidism the haemoglobin concentration was less than 12.0 g/dl in 37 of 207 women and below 13.0 g/dl in 9 of 32 men. Although some of these patients with a low level of haemoglobin were iron deficient, with a transferrin saturation less than 16 per cent, many were not. On treatment of hyperthyroidism the haemoglobin rose by an average of 0.5 g/dl in patients who had not been anaemic on diagnosis. A small fall in haemoglobin is therefore usual in hyperthyroidism and it may sometimes be sufficient to cause a mild degree of anaemia.The mean corpuscular volume (MCV) was decreased in hyperthyroid patients who had neither anaemia nor a reduced transferrin saturation. After treatment of hyperthyroidism the MCV rose in these patients by an average of 6 fl. A dimunition in MCV, even within the normal range, is an invariable concomitant of hyperthyroidism. The administration of thyroxine to excess does not, however, cause the same change in the MCV.On diagnosis of hyperthyroidism the prevalence of pernicious anaemia in this series of patients was 1.9 per cent. Gastric parietal cell antibodies were present in 15.4 per cent and thyroid microsomal antibodies in 49.5 per cent.
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