Is it permissible to break the fast in Ramadan due to thyroid disease, extreme thinness, and fear of complications such as hospitalization? And is fasting in this case forbidden?

There is no harm in breaking the fast if fasting results in harm or an increase in illness; in fact, breaking the fast may become obligatory. A sick person for whom fasting is harmful is forbidden from fasting, due to the Almighty's saying: "And do not kill yourselves. Indeed, Allah is ever Merciful to you" and His saying: "And do not throw [yourselves] with your [own] hands into destruction," and due to the Prophet's (peace be upon him) saying: "There should be no harm or reciprocating harm." Harm is identified by the patient's feeling or by the report of a trustworthy doctor. The patient must make up the days they missed after recovery. If the weakness persists and recovery is not hoped for, then feeding one needy person for each day becomes obligatory.