As can be seen from the photographs in the Hypothyroidism With Normal Blood Test Results topic I was severely hypothyroid even though I had excellent thyroid hormone levels.
I read a letter to the BMJ (page 1764) that described my condition and asked my GP for a referral to the author.
I was started on 25 mcg levothyroxine (L-T4) which gradually increased to 360 mcg L-T4 + 15 mcg L-T3 over a period of four years. Most signs and symptoms resolved but I still had an impaired memory and inability to concentrate. The treatment gradually migrated to 120 mcg L-T3 which gave a substantial improvement in cognitive function. After a few years some signs of thyrotoxicity appeared and the dose was reduced to 90 mcg L-T3.
These doses were supraphysiologic as confirmed by thyroid function tests (TFTs) showing high thyroid hormones and very low TSH. Severe clinical hypothyroidism with normal blood hormone levels followed by clinical euthyroidism with supraphysiological hormone requirements confirm a form of peripheral resistance to thyroid hormone (PerRTH). Adult onset followed by a subsequent reduction in hormone requirement strongly suggest this is not a genetic disorder. I coined the term ‘Acquired Resistance to Thyroid Hormone’ (ARTH) to describe this class of thyroid disorders.
After being on 120 mcg L-T3 for some time I was referred to the specialist centre at Addenbrooke’s Hospital. They ruled out the genetic form of RTH. I had a bone density scan which gave a ‘Z-Score’ of 1.9 which is good, very high hormone levels hadn’t affected my bone density. They determined my metabolic rate overnight using a special hood that measures oxygen consumption and taking measurements as soon as I wake up. This was messed up by a nurse that strolled into work half an hour late and perhaps affected the result. My metabolic rate was 115%, about 15% high which is not much for someone taking about three times the normal dose of thyroid hormone.
To understand what is happening we first need to look at the syndromes of Impaired Sensitivity to Thyroid Hormone.