Your Liver Blood Tests Decoded
What Your Liver Blood Tests Actually Mean (And What Your GP Might Miss)
If you've ever had a blood test come back with a flag next to your liver enzymes and been told everything is "fine" with no further explanation, you're not alone. Liver function tests are one of the most routinely ordered panels in medicine, and also one of the most misunderstood.
Here's the thing: they're not actually measuring liver function at all. They're measuring liver injury. And once you understand what each marker is telling you, you can start asking much smarter questions.
Let me walk you through what I look at, and why.
Before anything else: testing conditions matter
Results are only meaningful if the test was done correctly. Always fast beforehand, stay well hydrated, avoid intense exercise for at least 48 hours (even a single gym session can acutely raise your markers), and avoid alcohol for a full seven days. Non-fasting samples can artificially raise ALT. These details matter enormously and they rarely get mentioned.
Reference ranges also vary between labs, and they differ between men and women. Men tend to run higher across the board because of greater muscle mass, higher androgen levels and a higher red blood cell count. When I'm reviewing a woman's results, I'm using female-specific optimal ranges, not just the blanket lab reference.
ALT: the one to watch first
ALT (alanine aminotransferase) is the most liver-specific enzyme we can measure. It lives predominantly in the cytoplasm of liver cells, so when it shows up elevated in the blood, it means liver cells are under stress and spilling their contents. Think of it like a pressure valve giving way.
For women, the reference range is roughly 9-35, but the optimal window I'm looking for is 13-19. For men, optimal is around 13-29.
A raised ALT in women can point to fat accumulation in the liver even when it's not yet visible on imaging, insulin resistance, elevated androgens (this is uniquely significant in women and can be an early PCOS signal), gluten exposure in those with sensitivity, alcohol even in modest amounts, certain medications, or inflammatory conditions like RA or a viral illness.
Certain herbs can raise liver enzymes too, which surprises people. Green tea in high doses, black cohosh, kava kava and excess vitamin A are all worth knowing about.
One thing that often gets missed: ALT requires vitamin B6 to be synthesised. Alcohol depletes B6, which means heavy drinking can actually lower ALT by removing the building block. A deceptively low ALT in someone who drinks regularly is not reassurance.
AST: when the mitochondria get involved
AST (aspartate aminotransferase) sits in both the cytoplasm and the mitochondria of liver cells, as well as in cardiac and skeletal muscle. So an isolated AST elevation doesn't automatically mean liver trouble. It could be intense exercise, muscle breakdown, or cardiac involvement.
What matters is the ratio of AST to ALT.
When ALT is higher than AST (ratio below 1), this points to early liver assault. Something is stressing the liver cells themselves, whether that's fat, androgens, or a dietary trigger. When AST is higher than ALT (ratio above 1), the mitochondria are now involved. This is a later-stage response, pointing to metabolic dysfunction and deeper cellular energy impairment.
For women aged 20-60, a normal AST:ALT ratio sits around 1.15-1.7. When ALT is climbing and that ratio is inverted, I take notice.
ALP and GGT: bile, bone and oxidative stress
These two get grouped together because they both sit in the biliary ducts, but they tell quite different stories.
ALP (alkaline phosphatase) prompts me to think about three things: bile, bone, and barriers.
When ALP rises alongside GGT, the conversation turns to bile flow and the health of the biliary tree. When GGT is normal but ALP is raised, I'm thinking about bones. Bone turnover accelerates in late perimenopause and menopause, during growth spurts, in osteoporosis, and with vitamin D deficiency. ALP can actually serve as a useful marker of bone loss during the menopause transition. It rises as bone mineral density falls.
ALP also decreases with deficiencies in zinc, magnesium and vitamin C. If I'm seeing low ALP alongside persistent breakouts, low zinc is often part of the picture.
GGT (gamma-glutamyl transferase) is all about glutathione and oxidative stress. GGT sits on the surface of epithelial cells, particularly in the lungs, liver and gut, where it intercepts circulating glutathione, breaks it into its components, and recycles it back into fresh glutathione inside the cell. When the body is under oxidative load, GGT production goes up.
Optimal GGT for women is under 19. Standard lab ranges run much wider, which means many people are told their GGT is fine when it's actually signalling something worth addressing.
A raised GGT with normal ALT points to oxidative stress, adiposity, alcohol, smoking, heavy metal or pesticide exposure, and inflammatory conditions like RA or hyperthyroidism. Medications including methotrexate and paracetamol raise it too.
In Chinese medicine, the liver is responsible for the smooth flow of qi throughout the body. A liver under oxidative or metabolic stress creates stagnation, and that stagnation shows up in hormones, digestion, mood and menstrual health. These numbers are rarely just numbers on a page.
Bilirubin: red blood cell turnover and the liver's processing capacity
Bilirubin is the end product of breaking down haem from red blood cells. It starts life as a fat-soluble, neuro-toxic substance and must be converted to a water-soluble form by the liver before it can be excreted. This is why neonatal jaundice is taken so seriously.
The conversion happens via the UGT enzyme in the liver, which attaches a glucuronic acid molecule to bilirubin, making it water-soluble so it can travel out through bile, into the gut, and eventually out in stool (which gives it the brown colour) and urine (which gives it the yellow).
When bilirubin builds up, it matters which type is rising. High unconjugated bilirubin points to increased red blood cell breakdown or impaired conjugation. This is also the mechanism in Gilbert's syndrome, a very common and largely benign genetic variant that reduces UGT1A1 enzyme activity. High conjugated bilirubin points to a problem further along: liver cell dysfunction, bile duct obstruction, or more serious pathology.
To support the UGT pathway, the most potent dietary tools are cruciferous vegetables. The glucosinolates in broccoli, kale and rocket are the strongest dietary inducers we know of. Citrus fruit, berries, green tea, curcumin and resveratrol all support it too. Alcohol significantly impairs it.
A summary worth keeping
ALT up: early liver assault. Think fat, androgens in women, alcohol, gluten, inflammation.
AST up and higher than ALT: mitochondria involved, later-stage stress. Check muscle activity too.
ALP up with GGT up: bile and biliary issues.
ALP up without GGT: think bones, especially in perimenopause.
GGT up: oxidative stress. Think glutathione, toxin exposure, adiposity.
Bilirubin up: look at red blood cell turnover and UGT function.
If something is flagged on your results and you've been told it's fine without any explanation, it's worth a closer look. These markers, read together, are a detailed map of what's happening inside your liver and biliary system. And with the right dietary and lifestyle support, you can move them in the right direction.
Ready to understand your blood results in the context of your full health picture? Book a consultation at Vale of Health here.