Is a blood test the definitive answer as to whether a person has a thyroid disease?

It depends: Blood tests tell many things about the thyroid, including whether it's overactive (hyperthyroid) or underactive (hypothyroid). They detect anti-thyroid antibodies as well. These things cannot be confirmed without blood tests. Other thyroid diseases, such as nodular disease and cancer cannot be identified alone by blood test but require physical exam, possibly ultrasound, and possibly biopsy.

Thyroid: Yes. This will give a definitive answer whether you have thyroid disease or not.