What might cause a drop in T3 but an increase in T4?

[NatChap]

Just wondering what could cause T3 to drop but T4 to rise if meds remain the same?

[Hillwoman]

Reduced conversion ability. This could be due to other illness, nutrient deficiencies, or the deiodinase enzymes no longer working well. I've read various explanations for the cause of the latter, but it's difficult to find out in an individual case. The only remedy is to add some T3 replacement to the T4, but if you're not feeling any ill-effects, you may wish just to keep a closer eye on your health.