Underactive Thyroid
"I have been on synthroid for approx 10 years.  Here are several points that I have learned:
1.  You may initially start feeling better in a couple weeks but It takes MONTHS to see the full effects.
2.  Depending on your TSH levels, your Dr should start you at a low dosage and gradually increase as needed.  Starting with too high of a dosage is shock to your body and will even make you feel more tired as your body is working overtime.
3. See a Physician who is very familiar with thyroid issues.  If you are not seeing positive results with family Dr. see an Endocrinologist!  Many of the negative comments I have read may be a result of your Dr not understanding this disorder.  
4. There are alternatives if this medicine is not working."