Hello,
I am sorry to hear about your chronic health issues, and the recent development of lymphedema.
Regarding your specific query, there is some evidence to link obesity with risk of developing certain types of new cancers. See reference below for an overview on this topic. However, lymphomas are not among the cancers that are classically associated with obesity. Please note that the reference below is to development of new cancers, and not specifically the effect of obesity on risk of recurrence. In general, there is some data to suggest that cancer patients with normal body weight may have slightly better clinical outcomes compared to their overweight and obese counterparts (but no such data exists for lymphoma patients).
Given the lack of strong data linking weight to risk of lymphoma recurrence, you should not worry about a higher risk of relapse. However, as a general health advise, it would be good to discuss actively with the endocrinologist to optimize the thyroid treatment and attempt to lower weight.
All the very best, and God Bless.
Reference:
http://www.cancer.gov/cancertopics/factsheet/Risk/obesity