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What causes normal free t4 but low tsh?

Just got tested and I have a free t4 of 1.6 (0.8-1.8) a total t4 of 13.9 (4.9-10.5) and a tsh of 0.4 (0.4-4.5).  My tsh is usually 3-5 and trends in the 5’s.

What would cause this to occur? I have hashis and am on 100 mcg synthroid one day and 88 mcg the next, alternating.

I know I am missing free t3.
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Avatar universal
It is very important to understand that thyroid med is not additive to prior thyroid levels.  That is because when you start on thyroid med, it drops TSH in response, which reduces stimulation of the thyroid gland to produce natural thyroid.   The net effect is very little change in FT4 and FT3 levels, until the dose is high enough to basically suppress TSH.  At that point further increases in dosage will start to raise your thyroid levels.  

You are correct that you are missing FT3, which is very important, since it regulates metabolic activity in cells.  You should always insist on being tested for both FT4 and FT3, every time you go for tests.  This is especially important also because typically  patients taking .  T4 med find that their body does not adequately convert the T4 toT3.  

In addition, hypothyroid patients are frequently deficient in Vitamin D, B12 and ferritin.  If not tested, then you should do so and supplement as needed to optimize.  D should be at least 50 ng.ml, B12 in the upper part of its range and ferritin should be at least 100 to assure adequate serum iron.  

Even more important than all that is how you feel.  Please review the following list from Mayo Clinic and tell us which, if any, of these typical symptoms you have.  

Fatigue
Increased sensitivity to cold
Constipation (need laxative or fiber)
Dry skin (need skin creme)
Weight gain (unexplained by diet andor exercise)
Puffy face
Hoarseness
Muscle weakness
Elevated blood cholesterol level
Muscle aches, tenderness and stiffness
Pain, stiffness or swelling in your joints
Heavier than normal or irregular menstrual periods
Thinning hair
Slowed heart rate
Depression
Impaired memory
Enlarged thyroid gland (goiter)
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2 Comments
I thought free t3 was the precursor that was converted into free t4 which is the. Active one? I always get it reversed.

I have too much energy and feel tense. Yet, tired sometimes. I don't have the usual symptoms I get when hypo like stiff joints and brain fog, slowed thinking.

I will go pay out of pocket to get a free t3 test and it will show im normal as usual. I tell you you will never feel like yourself ever again once you get thyroid disease.  
Gimel so you think I am hypotyhroid? With low tsg high free t4 and high total t4?
Avatar universal
Just because FT3 is in range doesn't mean it is optimal, or even adequate.   That is not a large dose of T4 med, so with your FT4 being at 80% of the range, I am suspicious that not enough is being converted to T3.  We'll know more when you get the FT3 tested.  If not ever tested for Vitamin D, B12 and ferritin that would be very useful to know also.  
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I test low vitamin d and sufficient and even high b12.
Avatar universal
Vitamin D affects how thyroid is used in cells, so it is very important to be at least 50 ng/ml.  B12 in the upper part of its range is good.  Low ferritin can cause symptoms, and it is also important for conversin of T4 to T3.  As mentioned ferritin should be tested and supplemented to get it to at least 100.
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But Gimel, if I didnt change my dose why dod my tsh go fro 3.5 to .4
Avatar universal
It wasn't clear that your  previous TSH of 3.5 was when you were on that med dose.   What was it before you ever started on thyroid med?

Regardless, I would not worry about it since TSH  is basically a useless test when already taking a significant dose of thyroid med.   Med dosages should be adjusted as needed to relieve hypo symptoms without going so far as to create hyper symptoms.   FT4 and FT3 are useful to monitor the effect of dosage changes on thyroid levels.  
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Avatar universal
Those tests were a waste of time. You need a full thyroid panel with both antibodies and RT3 join Adrenal fatigue and thyroid care and get your answers . Also you can buy or check out from your local library stop the thyroid madness revised edition. T4 by itself is useless you can add T3 .
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