Tried to resist, but here goes...
Whilst we are scientists and look for scientific evidence in everything we do, we must not forget that we are Clinical Scientists and the human body does not behave like the Large Hadron Collider where we can control everything except the variable that we are investigating. People are complex and variable and to complicate it more they take other drugs, and get other illnesses, and grow older by the second. So iron clad evidence for the interpretation of our results is a luxury we do not have (I notice that no one has yet questioned whether we can actually measure FT4 correctly!)
So we need to be practical
My (simple, but practical) view:
Need to measure TSH and FT4 together to assess thyroid function
FT3 is misleading in interpreting thyroxine replacement
I do not like to see TSH suppressed as the pituitary (and the rest of the body) thinks that there is over-replacement
I am comfortable if the FT4 is slightly raised (if the TSH is not suppressed)
And yes, ask the patient how they feel if the results are a bit confusing
My answers to Mohammad's exam question (ignoring the FT3)
1st patient: TSH < 0.05 mU/L, FT4 23.3 pmol/L and FT3 3.0 pmol/L:
Suggests over-repacement
2nd patient: TSH 0.17 mU/L, FT4 25.1 pmol/L and FT3 4.4 pmol/L:
Suggests adequate replacement
3rd patient: TSH < 0.05 mU/L, FT4 25.1 pmol/L and FT3 5.7 pmol/L:
Suggests over-repacement
-----Original Message-----
From: Clinical biochemistry discussion list [mailto:[log in to unmask]] On Behalf Of Rita Horvath
Sent: 11 March 2013 22:10
To: [log in to unmask]
Subject: Re: Thyroxine over-replacement
Well, Nick - so TSH is no good for monitoring for reasons already mentioned rightly by John Martin.
fT3 is considered below as waste of time...
fT4 is useless as you are measuring the drug conc. at variable times in relation to when it was taken, plus it says nothing about the biologically active form and more importnatly about the tissue effects.
So, if none of the above are OK, then what do we need to measure to guide T4 Rx? Or shall we simply ask the patients how they feel on T4? NB: might be a better overall indicator than any lab tests?
BW: Rita
________________________________________
From: Clinical biochemistry discussion list [[log in to unmask]] On Behalf Of Nick Miller [[log in to unmask]]
Sent: Tuesday, 12 March 2013 5:20 AM
To: [log in to unmask]
Subject: Re: Thyroxine over-replacement
Well this panel thinks you are wasting your time worrying about circulating FT3 in patients on thyroid replacement.
Nick Miller
London
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