Please don't worry too much and panic at this stage , you did it for her benefit and I bet you she has thoroughily enjoyed her session with you.
If I was you I would check with your insurance provider to be on the safe side and get her to ask the gp or other healthcare worker ( they have regular blood tests) for their approval before the next treatment.
Sounds like hers was overactive and maybe pressing on her trachea or oesophagus.
I have done treatments such as a therapeutic back massage on my clients wo do have an underactive thyroid and it has not been a problem, as you say she is medicated and looked after by the gp so you should be ok to proceed.
( Both my mum and sister have an underactive thyroid and are on thyroxine and there are no treatments that they have missed out on in life. They have had Aromatherapy, Reflexology , Indian head massage and physiotherapy with accupuncture. None have been contra indicated to them.. It's hereditary in their case. However my maternal grandma had a complete removal, like your lady for no reason other than it being overactive. So her threatment was thyroxine too. )
All this will be noted on her consultation card anyway.
One point to mention is that she could well be at increased risk of osteoporosis so ease up a bit on the pressure if over her joints or vertebra just to be a little more gentle.
Have faith and check your insurance when you can.