If i was underactive thyroid blood test, what symptoms should i be watching out for?

Probably none.: Most cases of hypothyroidism are picked up by routine blood testing, in which thyroid hormones (t3 (liothyronine) and t4) start dropping, and thryroid stimulating hormone (TSH) start rising from a little gland in the head (the pituitary or master gland). Glands function on a feedback mechanism. If you have symptoms (lethargy, wt gain, dry skin/hair, constipation + 50 others), the DX is late.