can hyperthyroidism be difficult to diagnose? I have many many thyroid nodules, and consistant low THS .17 but t3 is usually ok and most recently my t4 was only .9 this is a week following an episode of tachycardia that lasted hours. I have had fertility issues, crazy red itchy eyes, anxiety, tachycardia, nervousness and bouts of diarrhea that don t seem to have a cause. Because My recent testing is only borderline for T4 my doctor feels the tachycardia was not related to the thyroid. but could not t4 have been even lower when the tachycardia happeended. the test was a week later.