Is it permissible for a patient with an underactive thyroid, who takes Levothyroxine, to break her fast in Ramadan if fasting causes her severe fatigue that prevents her from performing her daily duties and acts of worship?

Mere fatigue, tiredness, and lethargy do not permit breaking the fast. Rather, one must be patient and seek reward from Allah, for the reward is commensurate with the hardship. However, if the illness worsens due to fasting, or if recovery is delayed because of it, then this permits breaking the fast after consulting a trustworthy Muslim female doctor.