Thyroid function tests during substantial weight change Writing it up because I had to work it out twice and would rather nobody else did.
What changes if the standard account of Thyroid function tests during substantial is wrong? I ask because I have been treating it as settled and I noticed this week that I could not say why.
Working through the consequences rather than the evidence, since others here are better placed on the evidence.
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 would put the burden of proof on the interesting explanation, not the dull one.
This follows the opening post rather than contradicting it.
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.
A single observation, in a thread that deserves better than single observations.
Coming back to the opening post, because the follow-up matters more than the original answer.
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.
The answer changed when I changed how I was measuring, which was informative.
Post #5 is right about the mechanism and I think understates the practical bit.
I disagree with the framing of Thyroid function tests during substantial above, and I think it is a substantive disagreement rather than a terminological one. Setting out why, so it can be checked.
The reasoning depends on an assumption that is doing a lot of work and is never stated. If the assumption holds, the conclusion follows. I do not think it holds generally.
This settles it for me, at least until somebody posts a reason it should not.
Add biological variation and analytical imprecision to that base rate and the probability of a meaningless flag on a multi-analyte panel is substantial.
The arithmetic in post #9 is right; the assumption feeding it is the part to check.
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 checked the source rather than the summary, and they differ.
Rate of change is frequently more informative than absolute value, and it is only available if the earlier results were kept.
Adding the measurement that post #10 says would settle it.
Careful with the language on Thyroid function tests during substantial. "Not detected" and "not present" are different findings and the first is a statement about the method.
Post #12 describes the usual case. This is about the unusual one.
Two questions I would want answered before drawing anything from the Thyroid function tests during substantial data above: how were the cases selected, and what happened to the ones that dropped out.
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.
The practical version of Thyroid function tests during substantial is three sentences long. The rigorous version is three pages and reaches the same conclusion with the conditions attached.
Picking up post #14: that is the part I would want checked first.
I would keep Thyroid function tests during substantial and the decision it usually gets used for separate in this thread. They are related and they are not the same question, and merging them is why the last one went badly.
Post #18 answers the question as asked. The question underneath it is different.
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.
I would want the raw data before agreeing with my own summary of it.
I read post #16 twice before replying, because I had assumed the opposite.
Where I would push back on the Thyroid function tests during substantial consensus is the confidence, not the direction. The direction looks right. The confidence is borrowed.
Post #22 describes the usual case. This is about the unusual one.
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.
Adding the measurement that post #22 says would settle it.
Thyroid function tests during substantial has been discussed here with more heat than it deserves, mostly because two definitions have been in play the whole time.
Repeat before acting is standard practice for an isolated abnormal result on most analytes, and it is standard for a reason.
The part I am sure of is shorter than the part I have written.
Lipid changes during substantial weight loss are expected and their trajectory is not always monotonic. A single mid-course panel can be misleading.
Genuinely open to being wrong about this one.
The arithmetic in post #25 is right; the assumption feeding it is the part to check.
Two claims get bundled together under Thyroid function tests during substantial and they need separating. The descriptive one — this is what was observed — is usually well supported. The causal one — this is why — usually is not.
Almost every disagreement in threads like this one dissolves once you say which of the two you are making.
Post #25 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.
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.
Written quickly, so the reasoning may be tighter than the wording.