Endocrine Abstracts

Introduction: Hypothyroidism is a well-recognised cause of macrocytosis, but microcytosis is not widely recognised to be associated with hyperthyroidism. Thyroid abnormalities are often associated with various haematological changes and Graves’ disease has been reported to be mimicking beta thalassaemia trait in the literature. We discuss two cases of microcytosis related to hyperthyroidism that fully resolved once euthyroidism was achieved.

Case 1: A 24-year-old male with a few years history of hyperthyroidism on intermittent Carbimazole use was referred with relapse (suppressed TSH and elevated Free T4, 48 and Free T3, 18 pmol/l). He was incidentally noted to have microcytosis (MCV 76.1 fL, Hb: 135 g/l). In addition to starting Carbimazole, Ferritin and haemoglobinopathy screen were checked and these were normal. Once he became euthyroid his MCV normalised to 85.4 fL. He again became hyperthyroid (Free T4 66 and free T3 30) a year later and his MCV dropped to 79 fL.

Case 2: A 37-year-old female with symptoms suggestive of long standing hyperthyroidism was diagnosed with Graves’ disease (suppressed TSH and Free T4, 45 pmol/l) and was noted to have longstanding low MCV 75 fL. Her ferrin was normal at 23 µg/l and haemoglobinopathy screen was suggestive of delta beta thalassaemia trait. Interestingly after achieving euthyroidism her MCV normalised to 80.5 fL. We are planning to repeat her haemoglobinopathy screen to check if all the abnormalities have resolved.

Discussion and conclusion: Microcytosis is one of several well described but often under recognised haematological consequences of untreated hyperthyroidism. Thyroid hormones have been known to have a key role in regulating bone marrow haematopoiesis and iron utilisation by erythrocytes through a variety of poorly understood mechanisms. Ineffective erythropoiesis with reduced Iron utilisation could explain microcytosis. This change is noted to be more common in those with prolonged hyperthyroidism as in our cases. Incidental microcytosis with hyperthyroidism resolves with treatment and further investigations would only be needed if microcytosis persists after achieving euthyroidism.