Thyroid function tests during substantial weight change

Bookmarking this. I will come back when I have something worth adding.

Building on post #44 rather than restating it.

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.

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.

The rule of thumb is fine; the edge cases are where it earns its keep.

Post #72 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.

Posting it because the silence on this was starting to look like agreement.

Adding the measurement that post #101 says would settle it.

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.

I have deliberately not rounded that, because the rounding is where the argument starts.

Post #111 put the caveat in the right place and I want to underline it.

Small correction to my own earlier position on thyroid function tests during substantial. I had the units the wrong way round, which changes the conclusion by an order of magnitude and therefore changes it entirely.

Adding the measurement that post #126 says would settle it.

Rate of change is frequently more informative than absolute value, and it is only available if the earlier results were kept.

Checked the thyroid function tests during substantial claim against the primary source this morning. It survives, with a narrower scope than the version quoted here. Posting the narrower scope.

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.