Your body works best in a narrow band of temperature around 37°C, at which all your chemical apparatus is tuned for optimum performance. Your skin thrives best a little cooler, and your limbs are cooler to except in bed.
Occasionally you may use fever to out-run illness. Otherwise you stay close to normal so long as you eat enough food and your metabolism can burn it fast enough. Only a misguided starvation diet, liver disease, some medicines (beta-blockers used mostly in heart disease), over-exposure to cold weather and thyroid insufficiency are likely to prevent you.
Medical knowledge of thyroid function began to suffer in the middle of this century when chemistry replaced biology as the tool for investigation. Doctors have never since been able to stand back and see metabolism as a whole. Medicine consequently makes two false assumptions. Firstly, it associates thyroid function too closely with the levels in your blood of one or two chemical hormones supposed to control it. This is rather like using a fuel gauge and the maker’s performance figures to decide how far your car will run without refuelling: it takes no account of the conditions, or the quirks of your particular car. There is no substitute for a trial run.
In your case that means some measure of the actual rate of your metabolism, based on how fast you produce heat or use up oxygen. These measures are slow and unreliable but it turns out that your body temperature when comfortably at rest gives a pretty reliable idea of your metabolic tick-over rate, which is largely determined by your thyroid gland.
Secondly, one purified hormone is now supposed to represent adequately the output of your thyroid gland, as if its other three hormonally active products only get into your blood by accident. It is more logical to supplement thyroid function with whole thyroid glandular tissue, which is actually cheaper than the refined hormone. But tablets used not to keep well and their potency varied, so they were discarded. Their properties however, were different and longer lasting, especially in controlling blood cholesterol and preventing premature ageing of arteries — studies exist which show a ten-fold protection against stroke and coronary in especially susceptible people. The modern pure hormone tablets do not do this.
Two other factors can be brought to bear. In women it is common for progesterone not to be produced as it should, and in consequence your temperature does not rise in the second half of the menstrual cycle, when progesterone should be circulating. Replacement of this restores your sensitivity to your own thyroid hormones. If you are less than half a degree C below par, this may be enough to restore you to normal.
The second is a tip that comes from traditional Chinese medicine. Oriental doctors classify diseases in a different way, which includes groups of conditions due to an excess or deficiency of heat. There is a range of medicinal herbs available to treat deficiencies of heat, and these are a possible way forward if you cannot obtain thyroid or are vegetarian (thyroid is obtained from animal sources).
If you feel cold in conditions which toast most people, your doctor can quickly rule out all medically respectable causes. Even if he accepts what we say here, Thyroid BP is not easily available. You are on your own.
What to do