Because phentermine acts through sympathomimetic pathways, the drug may increase blood pressure and
heart rate. It may also cause palpitations, restlessness, and
insomnia. Additionally, phentermine has the potential to cause physical and psychological dependence."
If you are overmedicated with thryoid meds, you can become hyperthyroid, which does not seem like a good combination with phentermine (the symptoms for hyperthyroid are almost identical--heart palps, insomnia...).