Levothyroxine is the synthetic version of the human thyroid hormone. You might want to talk to your doctor about allergy testing as typically levothyroxine does not cause constipation and is generally very well-tolerated. It is possible that you have an intolerance to an inactive ingredient. There are no well-studied herbal products that are proven effective at treating hypothyroidism. Please make an appointment with your physician to discuss your concerns and possible treatment options.