I wouldn't panic yet. This may cause a delay in your donation, but I bet not an end.
Small masses (and .9cm is very small!) are not particularly uncommon findings on CT scans. Since you are a donor, your health is the top priority and the hospital is acting with extra caution. The referral to an endocrinologist is to rule out any hormonal changes cause by secretions from the growth. If you don't have any issues, then you'll be ready to go again.
Call around and tell your story - you may find an endocrinologist who will see you soon.