Can T3 make blood results hard to interpret?

[Tiredchick]

Just been prescribed T3 on top of levothyroxine, but my endocrinologist told me that introducing t3 can bake blood tests hard to interpret, whereas thyroxine on it's own is easier. Is this correct please?

[Jazzw]

Easier for him, maybe.

We manage around here just fine.

He's probably a slave to the TSH reading and worries about it being suppressed. Taking any amount of T3 often results in a suppressed TSH, but suppressed TSH in itself doesn't mean over medication, which is why FT3 testing is useful when someone's taking liothyronine, so you have a clue from the FT3 result whether someone's over medicating. Only a clue though. It's an imperfect science. Symptoms of overmedication, like racing heart, shakiness, losing weight very quickly etc are bigger clues, generally speaking.

[waveylines in reply to Jazzw]

Essentially he is correct. Blood tests for someone on T3 only are notoriously inaccurate. This is because your FT4 will drop as you are not taking any. The TSH will be suppressed be cause you are on a med contains T3. The Ft3 can and does fluctuate wildly -this is because your T3 level in your body naturally varies from hour to hour which is normal. T3 is the usable from of the thyroid hormones and is only in the body for a short period of time -roughly 5-6hours.

Therefore treating by blood tests using just blood tests is not very successful. Generally it has to be done using your temperature, pulse and signs/symptoms. It can be done very successfully. You might want to look at Paul Robinsons book called Living with T3 only and there is also an active support group on Facebook.

[shaws Administrator]

Well, the blood tests were introduced along with levothyroxine. So they 'match'. When T3 is added into the mix it will show a lower T4 level in the blood test and a higher T3 level. It is definitely not the blood tests which should dictate how we should be medicated it is how the patient feels that's the most important.

If we were overdosed we would have unpleasant symptoms (not that you don't have those when on levo mind you).

The main thing is to have thyroid hormones that suit us, i.e. makes us feel we have normal health again with no clinical symptoms.

[jimh111]

It is more difficult to interpret the blood tests when taking T3 containing medications due to the shorter half-life of T3 (around about 24 - 48 hours for liothyronine). If the blood is taken shortly after T3 medication the results go haywire. If you leave it too long (say 24 hours) the fT3 figure will be about half the 'real' average level, so you would need to almost double the fT3 figure to get a rought idea of your average level.

I take liothyronine twice daily. I take my bedtime dose and have the blood taken the next morning before taking my morning dose. I then add 10% to the fT3 figure to get a very rough idea of the 'real' figure. I think this is the best you can do to get an accurate result. The longer the gap between taking the T3 and drawing the blood the greater the error as T3 half-lives vary between people. Taking the blood within a few hours of ingesting T3 gives a silly result.

TSH is still a good indicator. The pituitary is exceptionally good at converting T4 to T3, better than other organs. So I can't see how T3 will have more effect on the TSH than T4. This study ncbi.nlm.nih.gov/pubmed/204... shows that T3 medication does not suppress the TSH. Interestingly the subjects took medication three times a day. It's possible that this is better than twice daily and that the spikes in T3 levels may have an effect on TSH (and also other organs).

So, if you take your T3 in divided doses your endocrinologist can still use TSH as a useful measure of dosage. T3 does not suppress TSH any more than T4 does, provided you do not take too much hormone.

But ..... I suspect most patients who require T3 need it either because they have some form of resistance to thyroid hormone or they have a TSH that is lower than it should be for given fT3, fT4 levels. TSH tends to promote T4 to T3 conversion. If TSH is low it will not fully convert T4 to T3. If more L-T4 (levothyroxine) is given the TSH falls even futher reducing conversion even more! TSH can be impaired due to depression, severe dieting or a sustained period hyperthyroidism (either by medication or perhaps undiscoverd hyperthyroidism from autoimmune thyroid disease).

So, it is more difficult to monitor. TSH will only be suppressed if you are taking suppressive doses! However, you may need doses that suppress your TSH in order to get better. In this case your endocrinologist should pay close attention to your signs and symptoms and titrate your dose accordingly. This is a poorly understood area of endocrinology. There are risks associated with a suppressed TSH and (greater) risks from remaining hypothyroid. It's a question of balance and risk management, this happens in other areas of medecine every day.

[Clutter]

Tiredchick, my endo doesn't have any problem interpretating my results on T4+T3. When FT3 was over range endo was happy for either T4 or T3 dose to be reduced and I decided to reduce T3 dose. TSH is suppressed because of the doses I take, FT4 is mid-range which is lower than it was on T4 only, and FT3 is just in the top third of range.

I leave 12-24 hours between last dose and blood draw and extrapolate FT3 results by +10% to estimate normal FT3 level if I leave 12 hours, or by +20% if I leave 24 hours.

[Angel_of_the_North]

Only if they aren't very good at interpreting blood tests! Both TSH and T3 rise and fall during the day, whereas T4 is more stable. But if they know you are taking T3 it shouldn't be a problem. Anyway, they should be treating you., not the lab tests (ha ha ha).