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hypothyroidism
I have recently been diagnosed with hypothyroidism.  My doctor performed a tsh 3rd generation with a level of 9.91.  He then ordered a t3 and t4 free assay, i do not have results yet. he started me on synthroid 25mcg. My question is what is the difference in the blood test as i am to go back in 6 weeks for a follow up blood test, and how do you know what would be the right dosage to start off on????  Please help.
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TSH is a pituitary hormone.  The pituitary is the master endocrine gland and controls all the other endocrine glands (including thyroid).  Many doctors think TSH is the gold standard when it comes to thyroid diagnosis and treatment.  However, many factors can affect TSH, so it really should be regarded as a screening test at best.  TSH is merely the "messenger" that your pituitary puts out to tell your thyroid to produce more hormone.

T3 and T4 are the actual thyroid hormones, and free T3 (FT3) and free T4 (FT4) are much more important indicators of thyroid status. T4 is the "storage" form of the hormones.  It basically floats around in your blood until your cells need thyroid hormone.  T3 is the "active" form of the hormones.  It's the only form your cells can use, and it must be converted from T4 before it can be used.  Conversion takes place in the liver, mainly, but it happens at various other sites throughout the body as well.

Starting dose is a bit of a guess.  It's usually based somewhat on weight and some on how low your FT3 and FT4 levels are.  It's often best to start out at a conservative dose (as your doctor is having you do), re-test in 4-6 weeks, re-evaluate symptoms and adjust from there if necessary.  

If you want to post FT3 and FT4 when you get them, we'll help you interpret them.  Be sure to get reference ranges along with results.  These vary lab to lab and have to come from your own lab report.

Best of luck...
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