Found the paragraph below on the site even more belower
A further complication of the use of T3 is due to the suppressive effect that it can have on TSH. The level of TSH influences the conversion of T4 to T3. A high TSH results in the maximum level of conversion of T4 to T3. A fully suppressed TSH will result in the conversion rate of T4 to T3 being reduced to the minimum level. For the patient on a combined therapy with T4 and T3 then this is very significant indeed. Some doctors attempt to perform simple mathematical calculations when they add T3 to their patient's T4. They reduce their patient's T4 dosage when T3 is added. Often there is no understanding that the added T3 is likely to have a suppressive effect on TSH and this is likely to downgrade any T4 to T3 conversion rate. Taking some of the patients T4 away as well is only likely to make things worse. So, finding the right balance of T4 and T3 can be an extremely challenging task.
Thoughts? It would appear to be a persuasive argument.