The thyroid releases several hormones, the main ones are T4, or thyroxine, and T3, or triiodothyronine. T4 is a storage hormone that converts to T3, which is the active thyroid thyroid hormone that enters the cells to maintain metabolic function. T4 converts to T3 when an enzyme removes one of its iodine molecules and this happens mainly in the liver.
TSH is thyroid stimulating hormone, which the pituitary releases when it senses that there is a need for more thyroid hormones. When thyroid hormones levels are high in the body the TSH levels will be low and vise-versa.
Generally if you are not converting well this can be from low ferritin and/or high or low cortisol. Many hypothyroid people have low ferritin and many docs don't test for it. Adrenal fatigue is also very common and it's essential to test correctly with saliva cortisol tests. High cytokine levels can also interfere with T4 to T3 conversion.
Many of us believe that T4-only hormone replacement isn't the treatment path we prefer to take. Requiring the body to convert all its T3 from what is essentially a storage hormone seems to cause additional stress and resultant adrenal fatigue and lingering hypo symptoms. Also our thyroids normally produce 5 different hormones which are necessary for metabolic health (one of which also aids in T4 to T3 conversion) so expecting a single storage hormone to be up to the task is not realistic. Some do ok on synthetic T3 and T4 others can only reach their best health on natural desiccated thyroid hormones, which contain all the thyroid hormones we normally produce.