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Thyroid UK
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I'm confused by this

Ok, I have read so much about not taking my thyroid meds prior to taking blood test in order to get a proper thyroid result.

I recently asked Dr.Westin the question below (actually this was my 2nd question).

My question:

Dr. Westin Why would taking your t4 and t3 meds prior to your blood test make your numbers worse instead of better ? I thought taking you thyroid meds prior to testing you would get you better results not worse.

Thanks, Grace

His response:

It has to do with the way that the medications raise serum levels of thyroid and how that influences the way that your doctor alters your dose in response to those changes. It’s a complex topic but I would recommend you read this post for more information which may help make sense of it: restartmed.com/hypothyroidism/

This topic can also be confusing for patients because they assume that blood levels perfectly represent the status of thyroid hormone in your body and it assumes that these tests are 100% accurate, neither of which are true.

I assumed by delaying my thyroid meds prior to blood test it would give a more accurate blood result or am I not understanding his explanation.

I will post 1st part of this question if needed.

7 Replies

First off, you need to define what you mean by 'worse'. Do you mean the values of T3 and T4 increase or decrease? 0r go over or under range? Blood tests are at best just a snapshot of your hormones at that particular time. They are never static, but are constantly fluctuating due to many factors.

Doctors unfortunately place huge importance on the TSH test value, and to a lesser extent, that of free T4. If you take a T4 dose close to your blood test time it will push your T4 up and your TSH lower. Your doctor will treat you based on your results, and this will almost certainly cause him or her to lower your Levothyroxine dosage. It's far better to leave off taking your daily dose till after your blood test, when the T4 you took the previous day will have had a chance to be metabolised.


It was my understanding if I took my meds prior to bloodwork my results would be (normal) because the medication is doing it's job and I thought if I delayed taking my meds until after I had blood test it would give a better idea what my meds are really doing. Hope this makes sense



Have just been looked at about a dozen medicines other than thyroid hormone which need to be precisely dosed.

Every single one that I read up on refers to taking a trough reading. That is, imagine a series of nice, neat waves on the sea. And what you need to do is measure true sea-level.

Obviously, if you measure the tops of the waves, that would be too high. It is only at that level at the very top of each wave. But you cannot realistically find any point on the rise or fall at which to measure - except the absolute trough.

That trough will be lower than "true" sea-level but is reproducible and makes sense.

That is what happens with FT4. You get a big wave in the hours after taking it, and a trough which is fairly flat afterwards until your next dose. If your own thyroid isn't making up the level, that trough will look a bit low. But how on earth can you sensibly choose any other point?

It would be great if we could slap a sensor onto our arms and continuously measure all these things. But we can't. At least, not yet.


Great response Helvella - giving a very clear picture of 'high and low'.

1 like

It isn't that they are worse or better, but that how they are interpreted may affect your Drs diagnosis and/or prescribing decisions. If you simply wanted to know the respective levels of your serum TSH/FT4/FT3 say, at the point of the blood being drawn, then the time of draw and how recently you took your thyroid hormones wouldn't matter, because it would just be a snapshot, of that situation, of those various factors, at that time. If you were to have bloods drawn on the hour every hour throughout a particular 24 hr cycle, without taking your thyroid hormones, you would have a picture of that day's levels. If you were to do the same on another day, but also take your thyroid hormones, you'd reasonably expect to see a difference from the first day's levels, but it wouldn't be worse or better, just a reflection of that day with exogenous thyroid hormone compared with the first day without. So then it all comes down to what you want to achieve - if you want to show your TSH at a daytime highest, to satisfy your Dr, you have blood drawn around 8am, fasting, and without having supplemented with exogenous thyroid hormone during the previous 24 hrs. Put simply, it's a game we feel we have to play ....


The main trouble is complete over reliance in a flawed testing system

Article just published by Dr Toft, past president of the British Thyroid Association and leading endocrinologist,


TSH daily variation


Median TSH graph



First, some patients can be hypothyroid with good blood levels of hormone. This is complicated and not part of this post, so I will not go into it.

The objective of the blood test is to get an accurate measurement of your average blood hormone levels. As far as thyroid hormone activity is concerned it is the 'free' hormone levels that matter, the portion available for cellular uptake and use. The much larger 'total' hormone pool does not matter for our purposes.

When you take a levothyroxine, liothyronine or NDT tablet there is a very large surge in your free hormone levels during the first few hours. As the hormone binds to proteins the free hormone levels settle down and then follow their normal half-life. The half-life of T4 is seven days and T3 has a half-life of about 24 hours. So if you are on levothyroxine only it doesn't really matter when you take the blood, as long as you give at least three hours after your latest dose. Provided you are not skipping a dose the result will be accurate to within about 10% which is good enough.

If you are taking any T3 medication (NDT or liothyronine) you need to have the blood taken about half-way between two doses, so that you get an accurate reflection of your average hormone levels (strictly speaking the average of the 'area under the curve').

Thyroid hormone is not like most other drugs, it controls metabolic rate and growth, it needs to be at the correct level. For example, if on an drug to control cancer one would want sufficient to control the cancer at all times, not let it grow for a few hours each day. Ideally with thyroid hormone one would want to be euthyroid at all times.

One technical bit, you can ignore if you want. Thyroid hormone has to enter the cell nucleus where it binds to thyroid hormone receptors and along with cofactors these attach to a 'response element' on DNA that causes the release of mRNA which leads to the production of proteins. We don't need to understand all this but just appreciate it takes time. When carrying out experiments on thyroid hormone receptors they are saturated in hormone for several hours before DNA expression takes place. So, although free hormone levels are very high after taking a tablet they do not have an immediate effect, it takes several hours on average. This is why we can ignore the very high hormone levels we have just after taking T3 medication.

So, if you want an accurate measurement of blood hormone levels it is best to have the blood taken about half way between doses. TSH is a good indicator of overall hormone levels in heatlhy subjects. TSH can underperform in some hypo patients and some patients need high doses of hormone that suppress the TSH because there appears to be peripheral resistance to thyroid hormone. You can only do your best to get an accurate blood test result. Once you have the results the real difficulty comes when you try to make sense of them!


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