Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice.
First think that I want to underline is to not quit the medication for low thyroid or thyroid
hormone replacement therapy because it is vital for your body.
Potential causes of pain in sternum and arms are: Reflux,
muscle pain, join pain, cardiac conditions like coronary artery disease, psychological conditions like panic and anxiety attacks.
From the description of your symptoms, in my opinion, Your problem may be caused by a muscle pain or it can have a psychological basis.
Levothyroxine can produce chest pain in patients with pre existent heart diseases, if it is started with high doses.
My advice is to consult your physician because it is necessary to do some basic tests like: an EKG, a chest and arms X-ray to exclude cardiac and joints conditions.
If the problem persist and the doctors do not find a physical cause, a
psychiatrist can be helpful.
Hope this information is useful. Let me know if you have other queries.
Regards.