Yes, you have a good case for poor conversion.
At 17.3 your FT4 is 53% through it's range of 12-22.
At 3.1 your FT3 is 0% through it's range of 3.1-6.8.
Unfortunately, the NHS rarely tests for FT3 and don't seem to attach any importance to it. The reality is that T4 is a pro hormone, it is inactive and some of it converts to T3 which is the active hormone that every cell in our body needs. Without a decent level of T3 we cant function well and remain hypothyroid.
From ThyroidUK's main website > Hypothyroidism > 'Treatment Options':
' According to the BMA's booklet, "Understanding Thyroid Disorders", many people do not feel well unless their levels are at the bottom of the TSH range or below and at the top of the FT4 range or a little above.
Dr Toft states in Pulse Magazine, "The appropriate dose of levothyroxine is that which restores euthyroidism and serum TSH to the lower part of the reference range - 0.2-0.5mU/l.
In this case, free thyroxine is likely to be in the upper part of its reference range or even slightly elevated – 18-22pmol/l. Most patients will feel well in that circumstance.
But some need a higher dose of levothyroxine to suppress serum TSH and then the serum-free T4 concentration will be elevated at around 24-28pmol/l.
This 'exogenous subclinical hyperthyroidism' is not dangerous as long as serum T3 is unequivocally normal – that is, serum total around T3 1.7nmol/l (reference range 1.0-2.2nmol/l)." '
Dr Toft is past president of The British Thyroid Association and leading endocrinologist. You can email Louise (as above) for a copy of the article to discuss with your GP.
Note that the article is talking about being medicated with Levo. When you add T3 then TSH and FT4 can lower, but again it is the FT3 result that is important.
It's also worth being aware that all nutrients should be at optimal levels for thyroid hormone to work properly, these are:
Vit D - 100-150nmol/L
B12 - very top of range, even 900-1000
Folate - at least half way through it's range
Ferritin - a minimum of 70, recommended is half way through it's range
If your GP won't test them then you can get them done privately with a fingerprick test as mentioned above.
Supplementing to achieve optimal levels then stopping supplements is a waste of time, once optimal then a maintenance dose is needed to stay there.
It's also worth testing for Thyroid antibodies to rule out (or in) autoimmune thyroid disease. There are two types - Thyroid Peroxidase (TPO) and Thyroglobulin (TG). The NHS rarely does TPO and almost never TG. Again these can be done privately.
There are a few general supplements that it's worth taking when hypo:
Vit C - 2000mg daily in divided doses, more if you want to bowel tolerance. This supports adrenals, thyroid and adrenals work together.
Selenium L-selenomethionine 200mcg daily - this aids conversion of T4 to T3
Mineral Complex that doesn't contain iron, calcium and iodine