? dead tissue

Hi - I have had a benign tumour on my thyroid for quite some time now. My biopsy 6 months ago showed large amounts of necrotic (dead tissue) and now my endocrinologist is sending me for further tests , a radio active isotope test and a repeat ultra sound scan. I dont understand the relevance of the dead tissue, or why the additional tests are required when the biopsy showed the tumor was benign. Does any one else know and if so could they explain? I am also slightly hypothyroid.

any info would be greatly received.