There is an odd asymmetry in how these drugs are followed up. Weight is measured obsessively, often weekly, sometimes daily. Almost nothing else is measured at all.
Meanwhile the person is eating perhaps a third of their previous intake, losing a fifth of their body mass, and taking a medication with recognised renal and biliary considerations. A twice-yearly blood panel costing very little would answer most of the questions the scale cannot.
The baseline panel
HbA1c. Establishes whether you have diabetes or prediabetes, and gives you a number to compare against later. A great many people in this population have undiagnosed prediabetes — which is also a qualifying comorbidity, so it matters for eligibility and coverage. What the number means and how far these drugs move it is in how much a GLP-1 lowers your A1c.
Lipid panel. Triglycerides in particular. Very high triglycerides are a pancreatitis risk factor, and they usually improve substantially with weight loss.
Liver function tests. Fatty liver is extremely common in this population, frequently undiagnosed, and improves markedly. It is also now a licensed indication in its own right — see the semaglutide liver approval.
Kidney function and eGFR. The most important one to have a baseline for, because dehydration on this drug class is the commonest route to trouble. See dehydration and your kidneys.
Full blood count, ferritin, B12, vitamin D. The intake-driven deficiencies. Ferritin especially in anyone menstruating — see what happens to your cycle.
Thyroid function, where there is a clinical reason. Note this is ordinary TSH, not calcitonin.
8 tests
A baseline panel worth having: A1c, lipids, liver, kidney, FBC, ferritin, B12, vitamin D
2×/year
Blood panel frequency that catches nearly everything worth catching
2 tests
Pancreatic enzymes and calcitonin — the ones not to run routinely
The two not to run
This surprises people, because a cancer warning and a pancreatitis warning both seem to imply surveillance.
Pancreatic enzymes — lipase and amylase. Mild elevations are common on these drugs and do not predict who will develop pancreatitis. Diagnosis requires roughly a threefold elevation plus characteristic symptoms. Screening an asymptomatic person produces incidental abnormalities, repeat testing and imaging, and catches essentially nothing. Test when there are symptoms — and the symptoms are distinctive, as set out in pancreatitis on a GLP-1.
Calcitonin. The prescribing information explicitly states that the value of routine calcitonin monitoring or thyroid ultrasound is uncertain. Calcitonin has a poor false-positive rate, and thyroid ultrasound in an unselected population finds nodules in a very large share of adults, almost all harmless. The reasoning is in the thyroid cancer warning.
What to expect the numbers to do
Most of these move in a good direction, which is a useful thing to have documented.
- HbA1c falls, often substantially
- Triglycerides fall, frequently by a lot
- Liver enzymes improve as hepatic fat clears
- Blood pressure falls — not a blood test, but it belongs in the same review, because medication doses often need reducing as a result
- Ferritin, B12, vitamin D may drift down on reduced intake
- eGFR may dip transiently during any dehydration episode and recover
That last point is worth understanding. A single low eGFR taken during a bad vomiting week is not evidence of chronic kidney disease. It is evidence of that week. Retest when you are well.
When to test more often
- Type 2 diabetes — HbA1c every three to six months, and more frequent glucose monitoring around dose changes, particularly if you take insulin or a sulfonylurea
- Chronic kidney disease — closer renal monitoring, and a lower threshold for checking after any illness
- On diuretics, ACE inhibitors, ARBs or SGLT2 inhibitors — the combination that most reliably turns a vomiting week into acute kidney injury
- On warfarin, levothyroxine or another narrow-window drug — monitoring around starting and each dose increase, per drug interactions
- After significant vomiting or diarrhoea — kidney function and electrolytes
- Persistent fatigue — iron, B12, thyroid, rather than assuming it is the drug. The expected timeline for ordinary fatigue is in how long side effects last
The scale answers one question badly. A twenty-dollar panel twice a year answers eight questions well, and almost nobody orders it.
Practical points
Fast if lipids or glucose are being measured — usually nine to twelve hours, water allowed.
Ask for a copy. You want the actual numbers, not “your bloods were fine.” A trend across three panels is far more informative than any single result, and you are the only person guaranteed to still have all three — which is the same argument as in tracking your GLP-1 journey.
Time it sensibly. Bloods drawn during a bad week after a dose increase will show that week. If you want a representative picture, test when things are stable.
Do not supplement blind. Correcting a deficiency you have documented is good medicine. Taking iron you do not need is not harmless — iron overload is a real condition. Test, then treat.
One place people have a haemoglobin measured outside a clinic is a blood donation centre, and being turned away there is clinically useful information — see can you give blood on a GLP-1.
The ask
At your next appointment: a baseline panel if you have never had one, and a plan for when it gets repeated.
It takes one sentence, costs very little, and turns a year of guessing into a year of knowing.