Phentermine works by suppressing appetite. How would that help you when you say you have no appetite and sometimes only eat one meal per day? I think it would be far better to get your metabolism optimal, by getting your thyroid levels optimal. As you mentioned, you are taking less T4 med than would be expected for your weight. In addition, doctors don't usually understand that hypothyroid patients should be medicated as needed to relieve hypothyroid symptoms, rather than just based on blood tests . They erroneously think that thyroid med adequate to return TSH within range is adequate. It is not. There is much more we need to discuss, but instead of repeating everything here, please click on my name and then scroll down to my Journal and read at least the one page Overview of a paper on Diagnosis and Treatment of Hypothyroidism: A Patient's Perspective. And then come back for further discussion..
I'm not going to say too much because gimel is going to lead you through the thyroid issue so you'll understand it very well. I just want to say that only eating one meal/day could be making your body conserve fat stores because you might not be eating enough. It's best to eat small amounts more times/day than a larger amount once/day.
Also, it's not necessarily the amount you're eating; it could have more to do with "what" you're eating. I had to stop eating breads, pasta and other items containing white flour and sugar, along with most dairy and add more healthful fats from foods like avocados, olive and coconut oils, etc in order to start losing weight, even after my thyroid hormone levels became normalized.
I love intermittent fasting. I just had this news article notification show up a few days ago..."Fasting diet 'key to weight-loss', Nobel Prize-winning study shows". You can google that if you want.