Thyroid desease what is it?

Different kinds. The thyroid can be underactive (make too little thyroid hormone), overactive, form a goiter (enlarged thyroid), develop nodules (growths, or masses), or become painful and tender. Each of these can have several different causes.
Affecting thyroid. Thyroid disease is a broad category which includes thyroid gland: autoimmune disease, cancer, iodine deficiency, congenital abnormalities. Many, but not all of these disorders affect thryoid function. These are all just examples. The thyroid gland is in the front lower part of the neck and helps to control metabolism, parts help calcium metabolism (calcitonin), also affects other gland sensitivity.