Is it just one foot and always the same foot?

hypothyroidism will usually have temp adjustment issue. They feel cold when others are okay. But if she is already on levothyroxine (Synthroid) and her TSH is normalized, she should not have the issue especially for the same single leg cold.

PVD (peripheral vascular disease) needs to be a consideration. Risks are hyperlipidemia, smoking and etc. She needs to have a good physical exam by her doctor and a test called ankle-brachial index (ABI) can be done to access the problem.

Good luck!