I’ve just added T3 or I’ve just started on thyroid meds – My heart races and I am having panic attacks. My TSH does not indicate that I am hyper. Why?

If your cortisol levels are a little low
(in the US  -  8AM  15 is good, below 12 is lowish)

Then your adrenaline and perhaps norepinephrine and epinephrine are often high??

Thyroid medication does sensitive the body to norepinephrine - i.e., you will respond more dramatically to norepinephrine when on thyroid meds.  Norepinephrine and epinephrine are the body chemicals that cause the palpitations, sweating, nervousness.  So, I'd get them checked.  Also, beta-blockers work on the body's response to norepinephrine. But some seem to take cortisol in small doses( 5mg of hydrocortisol 4 times per day is standard) and this stops this happening.

Having said that, I'd remove any and all caffeine from your diet as caffeine will also sensitize your body to norepinephrine. 

Another possibility is that you are not making enough thyroglobulin, the hormone that binds to the thyroid hormones and carries them around in our bloodstream.  Ideally, 80% of your thyroid hormone is bound to thyroglobulin and only 20% is free.  It is the free hormone that we use and that thus causes symptoms of hyperthyroidism.

Can you have your oestrogen checked?  Oestrogen changes the binding of thyroglobulin to thyroid hormone.  The higher the oestrogen level, the tighter the binding & thus the more difficult it is for the body to release from thyroglobulin.  If your oestrogen levels are on the low side, perhaps a little  oestradiol will help smooth things out.