Abstract: OBJECTIVE To explore the significance of clinical pharmacist in the pharmaceutical care of breast cancer patients with targeted therapy complicated with hypothyroidism. METHODS A breast cancer patient complicated with hypothyroidism was monitored for whole course medication by clinical pharmacist, and the use of mono antibody anti-tumor drugs and hypothyroidism drugs were analyzed. RESULTS Trastuzumab had an effect on the use of hypothyroidism drugs, resulting in an increased risk of medication and a reduction in the drug efficacy. Clinical pharmacists monitored the use of drugs. By reasonably adjusting the dose and usage of levothyroxine, the life quality of patient was improved and individual monitoring was realized. CONCLUSION For breast cancer patients with hypothyroidism, clinical pharmacists need to focus on the implementation of pharmaceutical services and assess the impact of disease and drug interactions to promote rational drug use.