Leptin resistance, inflammation, dieting, nutrient deficiencies such as low iron, selenium, zinc, chromium,
vitamin B6
and B12,
vitamin D, iodine, low testosterone, low human growth hormone, insulin dependent diabetes, pain, stress, kidney disease, severe or systemic illness, severe injury surgery, toxic metal exposure". So you can see there are a lot of possible causes, so that is why the best short term approach is to reduce the source of T4 that is converted to RT3.