We talk a lot about conversion problems here. They can be easy to spot in blood tests: if you're on a good dose of levo and your tsh has responded but it looks like your t4 is backing up (high) and t3 is low.
When I see a post that says something like 'I'm still symptomatic on a good dose of levo' I often comment that the op may still benefit from the addition of some t3 despite having no conversion problem. I feel like I should have a rubber stamp made.
My blood tests always showed good conversion, but despite what looked like an ideal t3 result (high in range) I was bloated, breathless, constipated, losing hair and exhausted. At the time I estimated that levo had given me a 20% improvement in my symptoms, which consisted of feeling slightly less sedated, slightly better feelings of desperation, not suffering w cold all the time.
A small addition of t3 made me feel loads better, and although I've had my ups and downs, a life w t3 is immeasurably better than a life on levo alone. My weight is more or less what it was before dx, my hair and brows have come back, I don't feel desperate or on the verge of tears all the time, I'm no longer breathless, gut situation is good (no more terrible constipation), etc.
I started to wonder why that is, why my tests looked good but I still felt awful on levo. There were so many concrete improvements after adding t3 that, although I'm open to the suggestion that it might be acting as a placebo, I didn't think that the sudden fluid loss and improvement in bowel function could be classed under the placebo effect. My mother also feels better on ndt (after being on levo for many years) so I imagined there could be some genetic reason we both need some t3 to relieve symptoms. So I found this article very interesting.
'The recent breakthrough discoveries described in the WATTS study reveal for the first time that individuals differ in how their bodies process (metabolize) thyroid hormone. While some may convert enough t4 to t3 in the cells of the body to restore normal function, due to genetic differences some individuals will not be able to make enough t3 leaving them with persistent hypothyroid symptoms. Since the problem is a deficiency of t3 within the cells of the body, measuring thyroid hormone levels in the blood cannot adequately reveal the problem. T4 replacement treatment alone can result in thyroid levels that appear normal on blood tests so doctors conclude that persistent hypothyroid symptoms are not related to the hormone therapy.'