A 34 year old woman has a follow-up telephone consultation with her GP after blood tests were requested due to worsening tiredness and frequent heavy menstrual bleeding. PMH. Hypothyroidism, migraine, gastro-oesophageal reflux disease. DH. Levothyroxine 100 micrograms PO daily, omeprazole 20 mg PO daily, sumatriptan 50 mg PO when required, mefenamic acid 500 mg PO three times daily during menstruation. SH. Works as a primary school teacher and lives with her husband and two children.

Investigations
Hb 102 g/L (115-165)
Platelets 355 × 10⁹/L (150-400)
Ferritin 9 micrograms/L (15-300)
Iron 7 micromol/L (10-30)

A diagnosis of iron deficiency anaemia is made and the patient is started on ferrous fumarate 210 mg PO once daily.

Select the most appropriate investigation to assess her response to treatment after 4 weeks.