Ive now been told by my doctor that i have an under active thyroid. what does this mean?

Hypothyroid: Hypothyroid is an under active thyroid, A condition called Thyroiditis can cause one's thyroid to be under active and depending on lab testing, you may need to take a thyroid supplement. Full lab testing includes Free T4, Free T3, (liothyronine) TSH and Thyroid Antibodies. Your doctor has most likely ordered these studies already. Depending on the results, he or she may initiate Levothyroxine therapy

Hypothyroid: Your thyroid produces two active hormones called T4 and T3 (liothyronine). They send a signal to the rest of the cells in your body to be metabolically active. When they drop or become ineffective you get sluggish and have symptoms of low, underactive or hypothyroid.