Blood tests for weight gain: what is worth doing
If weight has stopped responding, the useful tests go beyond a standard panel: insulin alongside glucose, thyroid function read as more than one value, sex hormones where relevant to your stage, cortisol assessed as a rhythm, and an inflammatory marker. Each answers a different question about why storage is being favoured over release.
There is a version of this question that is really a request for permission, to stop assuming the problem is effort, and start assuming there is something to find.
That is a reasonable instinct. Weight is regulated by identifiable systems, and most of them are measurable. The question is which measurements are worth the time and money, and what each one actually tells you.
What does a standard blood panel include?
Typically a full blood count, kidney and liver function, lipids, fasting glucose, and TSH.
That set is designed to do a particular job well: detect established disease efficiently across a whole population, at low cost. It does that job, and it is the right first step.
It is a narrower set than most people assume, though. It does not usually include fasting insulin, free T3 and free T4, thyroid antibodies, sex hormones, a cortisol rhythm, or an inflammatory marker such as HsCRP. Those are scope observations rather than oversights, a screening panel is not built to characterise borderline function in one individual.
Which tests are useful when weight will not shift?
The ones that describe how your body handles energy, rather than the ones that confirm you are not unwell.
In practice that means insulin as well as glucose; thyroid read across production, conversion and availability rather than at a single point; sex hormones where they are relevant to your stage of life; cortisol assessed across a day; and a marker of inflammation.
Individually, each of these is a partial view. Together they describe a control system, which is the only level at which a weight question can actually be answered.
Is there a test for insulin resistance?
There are several, and they answer slightly different questions.
The simplest is fasting insulin measured alongside fasting glucose. From those two values HOMA-IR can be calculated, an index that estimates how hard your body is working to keep glucose where it is. A person can have entirely normal glucose and a high HOMA-IR, which is precisely the situation a glucose-only panel cannot detect.
That gap can persist for years. Insulin tends to rise first, and glucose is often the last thing to move. Measuring only the last thing to move means finding out late.
Continuous glucose monitoring adds a different dimension again: how glucose behaves across a real day, in response to real meals, rather than at one fasted moment.
Why measure cortisol as a rhythm rather than a single value?
Because cortisol is supposed to change through the day, so one measurement cannot tell you whether the pattern is right.
Cortisol should rise sharply in the first hour after waking and decline across the day to a low point overnight. A single morning draw can sit comfortably inside its reference range while the shape of the curve is wrong, flat when it should be rising, elevated when it should be falling.
That shape matters because a disrupted rhythm promotes central fat storage and makes glucose harder to regulate. Assessed across a day, cortisol tells you something a single number cannot.
Does body composition count as a test?
It should, because it answers a question no blood marker does.
Knowing how much of your weight is muscle and how much is fat tells you something the scale cannot, and it changes both what a sensible plan looks like and how progress should be read. Weight holding steady while composition improves is a good outcome that the scale reports as a failure.
It is also the measurement that most often reframes the whole conversation. People arrive concerned about a number and leave understanding that the number was never the useful metric.
What is a sensible next step?
The point of testing is not to accumulate numbers. It is to work out which mechanism is operating, so that what you do next is aimed at the right thing.
More than 650 New Zealanders have completed an Autonomy clinical programme since inception.
Most people start with a Discovery Consultation, a conversation with a doctor about what you have been experiencing, what has already been looked at, and whether a fuller investigation is likely to tell you something useful. Sometimes it leads to testing. Sometimes it leads to a clear explanation without further investigation.
Dr Ula Heywood
MBChB, FACEM — Co-Founder and Lead Physician, Autonomy
