Objective and background: Anaemia and thyroid dysfunction are common and often co-occur. Current guidelines recommend the assessment of thyroid function in the work-up of anaemia, although evidence on this association is scarce.

Patients and methods: In the 'European Prospective Investigation of Cancer' (EPIC)-Norfolk population-based cohort, we aimed to examine the prevalence and type of anaemia (defined as haemoglobin <13 g/dl for men and <12 g/dl for women) according to different thyroid function groups.

Results: The mean age of the 8791 participants was 59·4 (SD 9·1) years and 55·2% were women. Thyroid dysfunction was present in 437 (5·0%) and anaemia in 517 (5·9%) participants. After excluding 121 participants with three most common causes of anaemia (chronic kidney disease, inflammation, iron deficiency), anaemia was found in 4·7% of euthyroid participants. Compared with the euthyroid group, the prevalence of anaemia was significantly higher in overt hyperthyroidism (14·6%, P < 0·01), higher with borderline significance in overt hypothyroidism (7·7%, P = 0·05) and not increased in subclinical thyroid dysfunction (5·0% in subclinical hypothyroidism, 3·3% in subclinical hyperthyroidism). Anaemia associated with thyroid dysfunction was mainly normocytic (94·0%), and rarely macrocytic (6·0%).

Conclusion: The prevalence of anaemia was higher in overt hyperthyroidism, but not increased in subclinical thyroid dysfunction. Systematic measurement of thyroid-stimulating hormone in anaemic patients is likely to be useful only after excluding common causes of anaemia.