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Does Cytomel (or generic T3 meds) influence Free T4 levels?

Thyroid labs were done 12/16------
TSH   0.605   (0.450 to 4.50)
Free T4   .99     (0.82 to 1.77)
Free T3    3.4    (2.0 to 4.4)

Four weeks prior my T4levels were at 1.3 (same reference ranges, but T3 was low doc suggested cytomel may help..
Well....4 weeks later, TSH is exactly the same, T3 is raised from 2.8 to 3.4, but t4 has DROPPED.
Endo says not to worry that T4 levels vary throughout day and there is no room to increase meds bc TSH is pretty low.
Anyone know if Cytomel infulences T4 levels?
5 Responses
Avatar universal
Adding T3 meds would only cause your FT4 to drop if 1) you still have some thyroid function left and 2) the T3 meds lowered your TSH.  TSH is variable intraday, so that may be the case.

Your TSH level should not be the determining factor in whether you have room for an increase or not.  FT3 and FT4 levels should determine that.  Your FT3 looks good; FT4 is a little low.

How do you feel?
Avatar universal
Here are my labs about 6 weeks PRIOR to the above post
Nov 2 labs
TSH   0.615   (0.450 to 4.50)
Free T4   1.3     (0.82 to 1.77)
Free T3    2.8    (2.0 to 4.4)
So...I believe I have thyroid function. Only thing different was the addition of the T3 (5 to 10 mcg a day). I wasnt aware that T3 could drop the FT4...

Avatar universal
Your thyroid needs TSH to stimulate it to produce thyroid hormone.  When T3 is added to meds, it often will lower the TSH.  With the lower TSH, your thyroid is not going to be producing as much T4 as it had previously.  Admittedly, it does not look like your TSH has gone down.  Were those labs both at approximately the same time of day?  

How much T4 do you take in addition to the 5-10 mcg of T3?  Do you split your T3 dose?
Avatar universal
Thanks so much for the input goolara.
My labs were taken in mornings with NO medication. (I wait to take meds after blood draw).
I take synthroid 50 mcg 5 days a week with a 100 mcg dose 2 days a week.
I take generic cytomel 10 mcg --dose split (1st dose 7am, 2nd dose at 2pm). Only been on the T3 meds for 4 weeks.

I will also add I am 12 weeks postpartum and the reason the endo suggested the T3 to begin with was b/c I was having swelling and extreme difficulty losing weight. (That symptom has since subsided with the onset of the T3 meds.) The befuddling factor is my T4 went from 1.3 to the gutter......
I almost wonder if the Synthroid should be reduced to my pre-pregnancy dose of 50 mcg 7 days a week?
Avatar universal
Well, wouldn't reducing your Synthroid just further reduce your FT4?  If you're post partum, your system is very much in flux.  You're rebalancing on all fronts!  I think I might give it a little more time before making any changes.  Are you feeling relatively well?
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