What is the fine-grained topic of the following text: However, in case of iron deficiency, thalassemia and abnormal porphyrin synthesis, the content of erythrocytes may remain normal (as to thalassemia, it may even be higher than normal), with a low level of haemoglobin. Only return the topic.
Iron deficiency, thalassemia, and abnormal porphyrin synthesis' impact on erythrocytes and haemoglobin levels.