[2026 update] Thyroid function tests during substantial weight change posts 31–60
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Collapsed as off-topic by two members at trust level 3 or above
Post #30 put the caveat in the right place and I want to underline it.
My experience of Thyroid function tests during substantial contradicts the reply above. I am posting it as a data point rather than as a refutation, because one person's experience is exactly that.
The arithmetic in post #31 is right; the assumption feeding it is the part to check.
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.
Not disagreeing with anyone above, just adding the bit I keep having to look up.
Answering the Thyroid function tests during substantial question as asked, then the question I think is meant. As asked: yes, with the qualification below. As meant: it depends on how the first measurement was taken.
Taking post #33 at face value and following it one step further.
The question underneath Thyroid function tests during substantial is usually "how would I tell?" rather than "what is true?", and that one has a method attached to it.
Write down what you would expect to see under each hypothesis before you collect anything. If they predict the same observation, collecting it will not help.
Post #34 and I disagree about the size of the effect, not about the direction.
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.
It is worth checking rather than assuming, which costs nothing.
This follows post #36 rather than contradicting it.
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.
Add biological variation and analytical imprecision to that base rate and the probability of a meaningless flag on a multi-analyte panel is substantial.
Narrowing post #39, because the general version has more than one answer.
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.
Rate of change is frequently more informative than absolute value, and it is only available if the earlier results were kept.
The confident version of this sentence would be wrong, so here is the hedged one.
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.
Worth reading the earlier posts in this thread before acting on mine.
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.
That is the version I use. It may not be the version that is correct.
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 holds for the case as described. Change the assumptions and it may not.
Where I part company with post #43, and it is a narrow parting.
The failure mode on Thyroid function tests during substantial is boring rather than dramatic. It is almost always the step everyone assumes was done correctly because it is too simple to get wrong.
Lipid changes during substantial weight loss are expected and their trajectory is not always monotonic. A single mid-course panel can be misleading.
Practical answer on Thyroid function tests during substantial, since the theoretical one is upthread: do the simplest check first, write down the result, and only then decide whether the complicated explanation is needed. It usually is not.
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.
The variance between people here is larger than the effect being discussed.
On post #52 — agreed on the reasoning, with one qualification.
On Thyroid function tests during substantial, I would rather understate and be corrected upward than overstate and be quoted. That is a house style here and it is a good one.
Confirming post #54 from a second method, which matters more than confirming it from a second person.
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 had written a reply contradicting post #52 and deleted it. Here is what survived.
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.
Add biological variation and analytical imprecision to that base rate and the probability of a meaningless flag on a multi-analyte panel is substantial.
Written in the hope of being told what I have missed.
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.
I am reporting what happened, not recommending it.
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.
Worth saying I have only my own numbers here, and n is small.
The reason Thyroid function tests during substantial is hard to answer is that the obvious measurement and the relevant quantity are not the same thing, and substituting one for the other is silent.