Hi,
I read everywhere on here that one should wait 6 - 8 weeks after dosage change to test thyroid levels. It is usually stated that it takes that long for levels to stabilize in blood.
Which levels take that long to stabilize - TSH, FT 4 , FT3 or all three? Do they stabilize at different rates?
Also, is this stability dependent on therapy - eg. does T4 mono take the same length of time to stabilize as T3 mono or T4 / T3 combination therapy?
Are there any studies to confirm this?
Thanks!
T4 has a seven day half life. So by way of example imagine if you have no thyroid and are started on 100 mcg L-T4. For simplicity we will only consider the dose on one day a week. After the first dose on day 1 you will have 100 mcg levo in your blood (you won't because absorption is about 40% - 60% but let's imagine it's 100%). One week later you will have 100 + 50 and the next week 100 + 50 + 25 and so on 100 + 50 + 25 + 12 + 6 + 3 ....
So, by six weeks your T4 level will have stabilized to within about 99% of its final level. In fact after four weeks it will be pretty stable and sometimes the first dose is adjusted after four weeks.
Another consideration is that although TSH responds rapidly to changes in fT3 and fT4 this is not so if TSH has been very low or very high. It can take many weeks to respond. This is another reason to wait six weeks before dose changes. Also, even if your hormone levels settle down quickly the metabolic changes they cause may take several weeks or months to take effect and for you to feel better. It can take quite some time for your body to recover once your hormones are at the correct level.
The six week interval is a guideline, I think a reasonable compromise. Doses can be adjusted sooner and in some cases doctors now start patients off on 100 mcg levothyroxine provided they are not over 60 or have cardiac problems.
T3 has a half life of 24 hours and so T3 levels will settle down within about a week but of course TSH and the body will take longer to respond.
Thanks jimh. I can understand the longer wait for LT4 mono, but when one is on Lt3 mono or NDT, why the long wait then? For example, my TSH is < 0.01. Nothing makes it budge and I've been diagnosed as Central Hypothyroid. My TSH is not a factor at all. I'm on NDT, so based on your explanation, I can understand waiting that long for the T4 portion to settle in, but not the T3.
I have heard some people on T3 mono reduce or even stop a few days before a test in order to bring their FT3 number down and TSH up. If it works within a few days to alter blood numbers by reducing, wouldn't the opposite be true as well?
PS. I'm on day 36 into a dose change. I felt better within a week or so of increasing, but now feel worse. Why?
TSH usually responds rapidly but if it has been suppressed for a long time it may fail to rise appropriately when hormone levels fall. You don't have to wait six weeks you can adjust your dose sooner.
I wouldn't advise reducing your dose just before a blood test, the figures you get are worthless and it can lead a doctor to believe you are now well on that lower dose. It just reinforces the belief that blood tests are infallible and sabotages are efforts to educate doctors.
You can feel worse a few weeks after a dose increase because your body reacts to the increase by reducing thyroidal secretion, reducing T4 to T3 conversion and increasing T3 to T2 conversion.
I take T3 alone when I awake in the morning with one glass of water.
If having a blood test - the earliest possible time - I don't take T3 until after the blood draw.
Thanks for your reply shaws. I don't take my ERFA until after my blood draw either. It's always around 10 hours from last dose.
Hi shaws is that 24 hours between taking T3 and the blood draw? I have started to trial taking my T3 all at once in the morning! Thanks
Yes. I hope you find T3 as helpful as I have.
Thanks Jim. My TSH has been suppressed "forever" - a long long time. It should have risen with some of the blood numbers I've had over the last 2 years, but just doesn't. I was diagnosed with Central twice; once several years ago and last year by a new endocrinologist.
I can't reduce my dose. I was just at 105 mg and had very bad stomach problems. Those have gotten a bit better with the increase, but it's just been replaced with worsened anxiety, vision problems and skipping heart beats. Overall, I feel awful. It just seems like it's always one thing or the other, and I can't find the sweet spot.
I think I should wait a bit and see what my numbers are before I increase again.
I guess I'm puzzled as to why I was on 150mg for about 20 years and now can't seem to tolerate 120.
Back to the initial question. Overall... TSH will generally stabilize quickly, unless it's been suppressed for a long time, in which case it can take a very long time. FT4 stabilizes slower than FT3 because of their relative half lives.
Are there any studies that support this?
My GP is refusing to retest until 3 months after levo dose change. Had my thyroid removed in May last year. Have had terrible fatigue since. Hospital put me on 75mcg then 6 wks later upped it to 100mcg. Now discharged to GP who waited 3 months to test then put me on extra 25mcg once a week because TSH out of range . 6 weeks have passed and I'm still fatigued but he won't retest until next month. Could take ages to get to correct dose 😒
I'm so sorry Brijenn. Maybe you need to try another GP? I have been very fortunate - my GP will test monthly while I'm trying to find my sweet spot.
Just testing TSH is completely inadequate
Suggest you get FULL Thyroid and vitamin testing done privately
Come back with new post once you get results
Approx how much do you weigh in kilo
Guidelines on dose levothyroxine by weight is approx 1.6mcg levothyroxine per kilo of your weight per day
But you might need more
Essential all four vitamins are optimal for good conversion of Ft4 (levothyroxine) to Ft3 (active hormone)
What vitamin supplements are you currently taking
Always test as early as possible in morning before eating or drinking anything other than water and last dose levothyroxine 24 hours before test
Which brand of levothyroxine are you currently taking
Do you always get same brand levothyroxine at each prescription
Many people find different brands are not interchangeable
List of private testing options
thyroiduk.org/getting-a-dia...
Medichecks Thyroid plus antibodies and vitamins
medichecks.com/products/adv...
Blue Horizon Thyroid Premium Gold includes antibodies, cortisol and vitamins by DIY fingerprick test
Slight Dose Change - help!
Feeling the effect of a dose increase after 8 hours?
Bloods after approx. 8 weeks of T3
How Long to Stabilise after Change of Dose
Why TSH only responds to change of circumstances after a long delay (if at all)
