Thyroid cancer is classified firstly into differentiated types or undifferentiated (also called anaplastic) carcinomas, secondly there is group of medullary carcinomas and lastly there are the lymphomas. Differentiated carcinoma is so-called because the cancer looks (down the microscope) like the thyroid gland tissue from which it has derived.
There are two types: the commoner papillary type (60% of all thyroid cancer) and the follicular type (17% of the total). The importance of distinguishing the diffentiated histology is that these are the types of thyroid cancer which retain the ability to concentrate iodine. If the iodine is made into a radioactive iodine isotope, then this radio-iodine is a tumour specific lethal weapon. It is the differentiated types of thyroid cancer that form the majority of cancers of this gland and correct management is attended by high cure rates.
Anaplastic or undifferentiated carcinomas of the thyroid retain little of the features of the original thyroid gland as discerned down the microscope. They tend to be faster growing and metastasising (spreading to other tissues) and have a worse outlook overall; furthermore, they do not concentrate iodine and so the radio-iodine option has no role in their management.
Medullary carcinoma of the thyroid is an unusual type of thyroid carcinoma completely unrelated to the other types. It is derived from the so-called C-cells which normally produce the calcium lowering hormone (calcitonin). If allowed to metastasise, it carries a bad outlook.
Thyroid lymphoma is almost invariably a high grade B cell lymphoma with a tendency to spread to other parts of the lymphoid system and bone marrow (see lymphoma section) a lower grade B cell MALT lymphoma, which has a lower tendency to spread outside the thyroid gland.