I get that question all the time. Most health care providers check a T4 and a TSH. If they fall within the lab's normal range, you're told you are normal. But there is more to it than that. There are other labs; there are "optimal levels" vs. "normal levels" and your thyroid function is tied to the rest of your endocrine system, especially your adrenal glands. I'll go through some of these things below.
TSH (thyroid stimulating hormone), which is produced in your pituitary, is not the most sensitive test. Free T3 is the best clue. There are also thyroid antibodies to be checked, T4, RT3, etc. But labs have to be clinically correlated with the patient. How a patient feels is the best indicator of what's going on.
Free T3 is the active hormone. T4 is a pro hormone. T4 must convert to the active T3 hormone. T3 is needed for fat loss. It protects against arrhythmias. T3 decreases with stress and aggressive dieting and with heart disease and congestive heart failure.
Synthroid (generic is levothyroxine) is a commonly used medication for hypothyroidism (underactive thyroid). It contains only T4. Replacing T3 + T4 provides many people a better quality of life, because many factors decrease T4 to T3 conversion including: dieting and stress, zinc, selenium and iodine deficiency, and certain drugs such as amiodarone, beta blockers and dilantin.
|Patient before and after thyroid replacement|