Higher cortisol levels reduce TSH (through the effect on the hypothalamic-pituitary system), and this reduces T4 to T3 conversion. Lower cortisol levels increase TSH. resulting in an increase in T4 to T3 conversion. Studies have found that in severe cortisol deficiency, the TSH and FT3 are often high. Cortisol (HC) supplementation in these people, normalises the TSH and FT3.