First thing is that your doctor seems to have the "immaculate TSH Belief" by which TSH is all they really pay attention to. This is very wrong. Except at extreme levels TSH is totally inadequate to use as the diagnostic for thyroid status. TSH has only a weak correlation with thyroid hormone and has only a negligible correlation with a person's thyroid status, and related symptoms. Diagnosis of a potential hypothyroid patient should start with a full medical history, followed by an evaluation for all symptoms, and then extended testing, to include thyroid hormones Free T4 and free T3 (not the same as Total T4 and Total T3), TSH, TPO ab if TSH is high, TG ab if TSH is high but TPO ab is normal, cortisol, Vitamin D, B12 and ferritin.
There is much to discuss, but before going further, please tell us about all symptoms you have. To make sure you don't overlook any, have a look at this link.
I have cryofibrinogenemia and if it gets colder so do my hands and feet. My feet were frozen and purple when it was 8C (46.4 F). I have livedo reticularis year round regardless of the weather.
"Skin manifestations are usually the first signs and are usually moderate; in addition, cold intolerance, Raynaud phenomenon, purpura, or livedo reticularis often occurs."
With untreated hypothyroidism my hands and feet were cold regardless of temperature. It's internally cold as metabolism is slowing down and body temperature decreases as well.
You said: "0.3 to 5.5 is a very big range. Is it possible even though my test result is within standard range, it is too low for me? If this range is so authoritative, then how about using thyroid supplement to raise my level from 0.644 to say 3.0, then I'm still within the normal range and won't have side effects, right?"
It's important to note that if you were put on a replacement thyroid medication, your TSH would be lowered, not increased, since TSH is counter to what one would think... TSH is a pituitary hormone produced to stimulate the thyroid when thyroid hormone levels get too low.
Typically, if TSH is low, thyroid hormone levels are higher and if TSH is high, thyroid hormone levels are lower. There are exceptions to this, however, which is why you need to have actual thyroid hormone levels (Free T4 and Free T3) tested, which is part of what gimel mentioned in his answer...
The very most important thing you need to do is find a good thyroid doctor, which is not easy. That does not necessarily mean an Endocrinologist, since they typically specialize in diabetes, not thyroid. They also tend to have the "Immaculate TSH Belief". If you will give us your location perhaps we can suggest a doctor somewhere near you.. In the interim you need to get tested for the actual thyroid hormones and others listed above. If you need info to help persuade the doctor to do those tests, click on my name and then scroll down to my Journal and read and copy the Overview for a paper on Diagnosis and Treatment of Hypothyroidism: A Patient's Perspective.
With that list of symptoms and your relatively low TSH it appears that you may have central hypothyroidism. With central, there is a dysfunction in the hypothalamus/pituitary system that results in insufficient output of TSH to stimulate the thyroid gland to produce needed thyroid hormone. Doctors usually overlook central hypothyroidism because of their fixation on TSH.
I'm in the Seattle area. Are there good thyroid doctors around? Thanks all for helping.
I am sending you a PM with info. To access, click on your name and then from your personal page, click on messages.
You may have Raynaud's Phenomenon. I have Raynaud's. It sounds like you may have it too. My hands and feet get ice cold with the slightest drop in temperature. They turn white, blue, purple at times. They tend to take forever to heat up to a normal, more tolerable temperature. As far as I know there isn't a whole lot they can do as far as finding relief. I wear gloves often. Good luck, I hope this helps you!! Take Care ; )