I've been progressing very very well since dumping T4 for T3, and gained grudging support from sceptical endos as I was so clearly improving, so I was somewhat devastated to hit an alarming wall 2 days ago - my first Hyper episode in 30 years. Last night involved palpitations, racing pulse, huge sweats ( complete change of bedding ) scrambling around for aspirin and plenty of time awake to wonder if my will was up to date. This morning I had an ECG and thorough cardio/pulmonary check -up at my superb GP's, bloods ( TSH free T4 free T3, thyroglobulin, plus (thanks for the insight, everybody) ferritin. B12 and cortisol ) What has changed ? Only one thing - I checked with Boots on a hunch, and until 2 days ago I was taking 60mcg MERCURY PHARMA liothyronine, on which I had no problems, and on collecting my repeat script, I was taking GOLDSHIELD liothyronine. I've reduced the dose to 10mcg of Goldshield at 3 per day until I can get more Mercury, and things appear normal. I can't draw any inference from this other than to recommend that you stipulate that each repeat script should come from the same supplier - Boots were fine with this - to avoid a scary and unpleasant episode. I reckon if I'd carried on I could have popped my clogs !