The dose of thyroxine depends upon the age and weight of the patient. In children, the dose is much higher as compared to adults. The adult dose is 1.8 ug/kg/d. But in elderly, we start with a small dose, i.e., 25 ug/d and increase slowly over a span of 4-6 weeks time. For your father, I suggest that you start with a dose of 25/d and after 15 days increase it to 50/d. Then get a repeat TSH after six weeks. If it is in normal range, then continue with the same dose. If it is still high, then increase it to 75/d. But we have to be cautious in your father's case as his BP is on the lower side and his Na is also low. In addition, I suspect he has cortisol deficiency, which can be tested by doing an 8.00 AM cortisol level.