What Does This TSH And Free T4 Value Indicate?

Posted on Tue, 16 Sep 2014

Medically reviewed by Ask A Doctor - 24x7 Medical Review Team

Tue, 16 Sep 2014 Answered on

Mon, 6 Oct 2014 Last reviewed on

Question : Specialist took me off carbimazole based on tsh reading only. Free t4 still high at 21. Tsh in normal range. Please explain.

Answered by Dr. Shehzad Topiwala (34 minutes later)

Brief Answer:
Thyroid

Detailed Answer:
That is a bit unusual. Free T4 is a good test to rely on most of the time. However whenever I suspect a mismatch between TSH and Free T4 I order the Total T4 and T3. Here in your case if the free T4 is not consistent with what a physical examination may suggest then it would be worth considering total thyroid hormone levels. TSH can lag behind free T4 meaning it generally takes a while to ride back up
into normal range even as thyroid hormone levels have begun to normalize. I would like to know what dose of carbimazole you were on

Above answer was peer-reviewed by : Dr. Chakravarthy Mazumdar

Follow up: Dr. Shehzad Topiwala (2 minutes later)

I was on 5mg daily. My free t4 peaked at 27 and lowered to 20. Tsh was never out of range.

Answered by Dr. Shehzad Topiwala (8 hours later)

Brief Answer:
Follow up

Detailed Answer:
I see. It was quite a small dose.

One possibility is that the free T4 laboratory test method may not be the most reliable one, unless it uses a technique called 'Direct Dialysis'. So you may wish to check on this.
If not, the alternative is to seek corroboration with Total T3 and Total T4 levels. If these are also high then a systematic re-evaluation may be necessary to thoroughly investigate this. If they are normal then confirms the impression that both TSH and thyroid hormone levels are normal, and it would be justified to stop carbimazole and retest in 4 to 6 weeks.

Above answer was peer-reviewed by : Dr. Chakravarthy Mazumdar

Follow up: Dr. Shehzad Topiwala (32 hours later)

Latest labs show tsh 1.8 free t4 25. Told to stay off carbimazole despite results of two weeks ago being tsh 3.9 free t4 20. Told my natural free t4 must be above range. Original results in XXXXXXX 12 tsh 1.2 free t4 27. Only ever symptom is pvcs. All cardiac tests echo eg are normal. My endo is unhappy with gp as she says I should never have been on carbimazole never mind for over two years.

Answered by Dr. Shehzad Topiwala (4 hours later)

Brief Answer:
Second follow up

Detailed Answer:
I follow what you are saying.

Again, to re-emphasize my previous explanation: Free T4 can be misleading at times. Therefore it is important to go by TSH and Total T3 and Total T4 and T3 Resin uptake. This gives an idea if there is truly any thyroid hormone excess in the body.
Interpreting thyroid labs can be tricky sometimes. Endocrinologists are specially trained in this and are experts in analyzing these. It can be difficult to explain it to the non-expert because it takes many many years of intense education to become an Endocrinologist.

Above answer was peer-reviewed by : Dr. Chakravarthy Mazumdar

Follow up: Dr. Shehzad Topiwala (14 minutes later)

So before I sign off and I do thank you for your time it is possible for me to have naturally high free t4 readings?

Answered by Dr. Shehzad Topiwala (8 minutes later)

Brief Answer:
Third follow up

Detailed Answer:
Well that is one way to put it. But it is more like a laboratory method is unable to measure the Free t4 in your blood accurately.

Above answer was peer-reviewed by : Dr. Chakravarthy Mazumdar