We do not see patient's with severe hypothyroidism often, but it is important that they be treated aggressively. Some treatment pearls are
Rule out aggravating cause (i.e.: infection [UTI, pneumonia], myocardial infarction)
Start IV levothyroxine dosing
Initial dose 400-500 mcg. This is a large dose but it only saturates the thyroid receptors and will not cause a rebound hyperthyroidism state.
Daily dose 100 mcg/day
Consider starting Dexamethasone/hydrocortisone
Patients may also have adrenal insufficiency from primary pituitary failure or may have secondary adrenal suppression due to the severe hypothyroidism. If dexamethasone/hydrocortisone is not provided they may develop severe adrenal insufficiency once you kick start their metabolism.