Thyroid problems

Diagnosed with underactive thyroid in 2000 but it's been well under control with medication until recently. My TSH has been going up since July, and my doctor increased my medication twice already. I just had TSH checked on Friday and it's up again. Anyone else have this problem while on herceptin or immediately afterward. I had a scan of thyroid and no tumors found, so I'm hard put to understand why the numbers are increasing despite a higher dose of synthroid. Any thoughts would be appreciated.