Posting this under the heading it deserves: Thyroid function tests during substantial weight change Everything below is what sits behind that.
I was wrong about thyroid function tests during substantial in a thread last spring and I would like to correct it publicly rather than quietly.
The error was in the units, which changed the conclusion by an order of magnitude. Setting out the corrected version, and the way I now check for that class of mistake.
Thyroid function tests during substantial is well covered in the tag pages, and the older discussions are better than the recent ones because they were argued out properly. Worth twenty minutes before adding to this one.
Biological variation: the person-to-person variation in a biomarker for a healthy person is larger than most people realize. Comparing your result to the reference interval is one thing; comparing your result today to your result from months ago is more sensitive to change.
Analytical imprecision: any measurement has an error margin. A small difference in consecutive tests is usually measurement noise, not a real change. Knowing the imprecision helps distinguish noise from signal.
The interesting part of this is the exception, and I do not understand the exception.
Nobody has said the unglamorous part of thyroid function tests during substantial yet, so: most of the variation is explained by things that are boring to write about and easy to check.
I read post #6 twice before replying, because I had assumed the opposite.
Time of day: some biomarkers vary across the day. Cortisol in the morning differs from cortisol in the evening. Comparing results from different times of day is comparing things that are not the same.
Small point, but it is the one that usually catches people.
I had written a reply contradicting post #6 and deleted it. Here is what survived.
Fasting state: some lab tests require fasting; others do not. Lipids on a fasting draw differ from lipids on a fed draw. Always note whether the test was fasting when comparing results.
I would put this at better than even and not much better.
A methods point on thyroid function tests during substantial rather than a substantive one: if the comparison is not like for like, the difference you are measuring is the difference in method.
I had written a reply contradicting post #9 and deleted it. Here is what survived.
A trend needs at least three points to be a trend. Two points are a line and a line through noise is still a line.
It is a small point and it changes the answer, which is an awkward combination.
If someone has run thyroid function tests during substantial properly I would rather read that than my own reconstruction of it. Posting mine only because the thread has gone quiet.
The arithmetic in post #14 is right; the assumption feeding it is the part to check.
Taking thyroid function tests during substantial seriously for a moment rather than deflecting: the honest position is that the community has observations and no controlled comparison, and those two things support very different sentences.
Renal function markers: eGFR is estimated, not measured. It depends on creatinine, age, and weight. A small change in creatinine might not mean a real change in kidney function.
The disagreement above is smaller than it looks once the terms are fixed.
Worth separating two things that post #17 runs together.
Ask what decision the result would change before ordering it. A test whose result changes nothing is a test that generates worry rather than information.
The number is defensible. The precision I gave it is not.
Acknowledging rather than arguing. The reasoning holds as far as I can follow it.
Lipid changes during substantial weight loss are expected and their trajectory is not always monotonic. A single mid-course panel can be misleading.
That has held every time I have looked, which is not the same as always.
Everything in post #21 holds. The case it does not cover is the one I have.
Having read the whole thyroid function tests during substantial thread before replying: the question in the first post has not actually been answered yet, and three of us have answered a nearby one instead.
Building on post #22 rather than restating it.
An honest declaration on thyroid function tests during substantial: I have a prior here and it is strong enough that you should weight what I say downward. Stating it rather than hiding it.
Post #22 answers the question as asked. The question underneath it is different.
Reference intervals: constructed to contain the central 95% of a reference population, which means one in twenty healthy people falls outside one by definition. Add biological variation and analytical imprecision and the base rate of a meaningless flag is substantial.
That is the version I use. It may not be the version that is correct.
Grateful for the specificity. Vague answers to this question are what sent me looking.
Thyroid function tests during substantial looks different depending on whether you are reading the primary literature or the summaries of it, and the difference is not in our favour.
Repeat before acting is standard practice for an isolated abnormal result on most analytes, and it is standard for a reason.
I have changed my mind on this once already, so take it as current rather than settled.
Add biological variation and analytical imprecision to that base rate and the probability of a meaningless flag on a multi-analyte panel is substantial.