The best possible way to know if a dose suits, is 'how we feel' on it. If you are improving but still have symptoms add another small dose every 2 weeks.
This doctor, and I wish most were trained like this one below, only took a blood test for the initial diagnosis then treated his patients until they were well.
Blood tests don't tell the doctor how the patient feels but they have their place if undiagnosed or if patient has taken too much (in that case they will not feel so good with palps or heart fluttering. (these two things also happen with too little but we do realise if it is too low if GP keeps us to a certain dose and we have no improvement.