Thyroid function tests during substantial weight change posts 121–150
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Source for the thyroid function tests during substantial figure, since it was asked for. It is in the discussion rather than the abstract, which is why the version circulating is stronger than the paper is.
Reading the surrounding paragraph is worth the two minutes. The authors are more careful than their summarisers.
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.
This is where my knowledge stops and I would rather mark the edge than blur it.
Practical note on thyroid function tests during substantial: write down what you expect before you look. The number of times I have found what I went looking for is higher than chance would allow.
Glycaemic markers integrate over different periods. A point glucose and a longer-term marker disagreeing is not a contradiction; it is two different questions.
I keep a log of this specifically because memory is unreliable about it.
I had read the opposite somewhere and cannot now find where, which tells me something.
Collapsed as off-topic by two members at trust level 3 or above
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.
Worth checking against a second source before it gets quoted onward.
Post #128 answers the question as asked. The question underneath it is different.
Adding the boring version of thyroid function tests during substantial, because the interesting version keeps getting posted and the boring one is usually right.
Check the ordinary explanations, in order, and stop when one of them accounts for what you are seeing. Most of the time the second one does.
TSH and free hormones: TSH is a reasonable screening test but does not tell you about actual free hormone levels. If TSH is abnormal, free thyroid hormones confirm whether there is a real thyroid problem.
I have written this out at length because the short version keeps being misread.
Narrowing post #130, because the general version has more than one answer.
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.
Collapsed as off-topic by two members at trust level 3 or above
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.
Written quickly, so the reasoning may be tighter than the wording.
The arithmetic in post #135 is right; the assumption feeding it is the part to check.
What I can speak to on thyroid function tests during substantial is narrow, so I will keep it narrow rather than generalising from it. Beyond that boundary I do not know.
That is a cleaner way of putting what I was circling around.
I read the earlier replies on thyroid function tests during substantial twice before writing this, because I had assumed the opposite and wanted to be sure I was disagreeing with what was said rather than what I expected.
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.
If that reads as pedantic, it is, and it has saved me twice.
Post #141 and I disagree about the size of the effect, not about the direction.
Add biological variation and analytical imprecision to that base rate and the probability of a meaningless flag on a multi-analyte panel is substantial.
A modest claim, modestly supported.
Taking post #141 at face value and following it one step further.
On thyroid function tests during substantial I would separate what is worth knowing from what is worth acting on. The first list is long and the second is short, and conflating them is how threads get heated.
Repeat before acting is standard practice for an isolated abnormal result on most analytes, and it is standard for a reason.
Nothing above should be read as advice about what anyone else should do.
A note on how thyroid function tests during substantial gets discussed rather than on thyroid function tests during substantial itself: the confident posts get the replies and the careful ones get ignored, and the careful ones have been right more often.
Post #145 put the caveat in the right place and I want to underline it.
Liver enzyme changes during rapid weight loss are commonly reported and have several possible explanations, which is exactly why they need clinical interpretation rather than forum interpretation.
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.
Same conclusion as the reply above, reached differently, which is mildly reassuring.
Adding the measurement that post #149 says would settle it.
Offering a way to settle thyroid function tests during substantial rather than another opinion about it. Two measurements, taken the same way, a fortnight apart. If the difference is within the noise, the question was not answerable at this precision.
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