Pediatric Endocrinology Expert Forum
T4 Levels Low but TSH normal
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T4 Levels Low but TSH normal

What exactly is low T4 for an infant age under two months?  I have a friend whose son's PKU came back slighty low is what they told her 6.5 I think?  But they are having her see an endocrinologist.  What could be the reasonings for the low T4? He was 2 weeks early but weighed around 7lbs going home.  She is definitely freightened thinking within her readings of Mental Retardation.

Please explain this to me....
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ok, so newborn screen often referred to by many as PKU-which actually is only one of the tests, pku is a test for phenylketonuria, the metabolic disease that prevents you from breaking down a substance(actually something that is in Aspartame and these kids have to have a modified diet for several types of proteins)
then other parts of the newborn test include thyroid, homocysteine, sickle cell and in some states 17-OH progesterone and others depending on the legislation-yes even newborn screens have politics!!
The thyroid was started cause many many newborns with hypothyroidism were being missed and this is a very important medical condition to treat so that we can optimize brain growth, if left untreated studies have shown lower IQ and developmental problems amongst other concerns. Needless to say we had to decide state by state how to screen for thyroid, some states choose T4, others choose TSH
The T4 is the level of thyroxine in the body-Total level, so that is influenced by protein binding. so if the T4 is low maybe this is not necessarily hypothyroid but several things-not enough blood on the paper, not enough TBG which binds the Thyroxine and leads to a false low etc.
So when a newborn screen has abnormal, the red light goes on and us endocrinologists get a call on the bat phone, everytime I am on call I know about EVERY patient in middle tennessee and abnormal screens,even the false alarms, I get alerts on a daily basis.
It is a safety measure that we have fought hard to become involved in so as not to miss any child who needs workup and potentially treatment
So congenital hypothyroidism-what they will do is repeat the level in the lab rather than on filter paper and get a tsh and a free T4, if the level is still out of range it is best to treat and repeat things so as not to lose time but it depends on the numbers and the baby.
normal T4 ranges vary from lab to lab
in a 2-6 week old there are many normal ranges
usually 6.5-16.3 micrograms/dl is normal range for total T4
But a baby who is newborn up to 4 days can have levels from 14-28 micrograms per dl since when they are first born the brain puts out a big signal to the thyroid getting it working and there is a "surge" of tsh from the brain and a higher initial thyroid level that comes down to usual ranges that we are used to in older children.
by 2-20 weeks the total t4 can range from 8-15 ug/dl
so ranges are based on the lab
referral to an endocrinologist is wise and please don't let them worry about his development at this time, have her focus on keeping the appt and having her questions written down and discuss them with the doc so she feels well informed and calmer about how healthy her baby is.
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