The major blood test that is used to screen for thyroid problems is something call the TSH test. This is probably the test that your doctor
ordered when you told them that you were suffering from symptoms of palpitations and anxiety, which can be associated with an over active thyroid gland, or hyperthyroidism. If the TSH test had come back very abnormal, then this would have been consistent with hyperthyroidism and would have required treatment.
However, what is much more common is for the test to come back just slightly abnormal. In this case, it is hard to say if the palpitations or anxiety might be from the thyroid, since they are more likely to be from another cause. In this situation, the slightly abnormal test might indicate that you are in the process of developing hyperthyroidism and might eventually need treatment. Many doctors
in this situation will choose just to "watch and wait" - following your symptoms and checking blood work periodically. They may also check some of the other thyroid function tests, such as T3 or T4, to see if there are abnormalities in those levels.
I would talk with your primary care doctor
about this issue again, to get a better sense of exactly what they are thinking.