"Isotretinion has been controversially shown to cause reversible hypo- and hyperthyroidism. 113-116. The effects of Vit A derivates on thyroid function still remain debating." From the book World Clinics: Dermatology - Acne by Neena Khanna, Raj Kubba
To gain so much weight in such a small period of time is likely to be just fluid (a symptom of hypothyroidism). There are 3 youtube videos (still more episodes he has to upload on his journey) on Diet Induced Hypothyroidism. He didn't eat enough and exercised too much and caused hypothyroidism.
Why not start by posting your current thyroid hormone levels, with reference ranges, which vary from lab to lab, so have to come from your own report. That will give us an idea of your actual thyroid status.
Also, please explain why your doctor feels that this is a "reversible" case of hypothyroidism... A brief research of Isotretinoin indicates that it "may" trigger an autoimmune response, however, once an autoimmune response has been triggered, it's NOT reversible... Once you have the antibodies, you can't make them go away and they will continue to destroy your thyroid...
The antibody tests you need are Thyroid Peroxidase Antibodies (TPOab) and Thyroglobulin Antibodies (TgAb)... You need them both because some of us have one or the other and some of us have both... If either are elevated it would the basis for a diagnosis of Hashimoto's.
If this doctor will not agree to treat your hypothyroidism, find one who will. Even if your hypothyroidism "does" reverse, you can stop treatment; in the meantime, you can't just continue being hypo as being hypo affect all of your body process, not just your weight.