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There are higher chances of hypothyroidism in a patient who has hypoadrenalism (Addison's disease). These both conditions can be the manifestation of the same underlying process (autoimmune). If your mother is symptomatic (weakness, dry skin, hair loss, constipation, pedal edema, etc.), then she requires thyroid supplement. To start it, I will need further investigation (free T4 level, anti-TPO antibody reports). Does she have any underlying heart disease? Please provide this information so I can help you in a better way.
For more information consult an internal medicine physician online --> https://www.icliniq.com/ask-a-doctor-online/internal-medicine-physician