Liver Function Tests: What ALT, AST and GGT Actually Measure

You get your check-up results back, scan down the page, and there it is in red: a raised ALT, maybe an AST or a GGT to keep it company. The label at the top of that block says "liver function tests," and your mind goes straight to the worst place. If you've recently arrived in Pattaya, or you fly in for a check between countries, there's a good chance no one has explained what these numbers actually are. So here's the honest version, in plain language: most of the markers on a "liver function" panel don't measure how well your liver is working at all — and a mildly raised one is usually telling a far less alarming story than it first appears. This is general education to help you read your own report, not a diagnosis; the numbers are interpreted by a doctor who knows your history.

Why "liver function tests" is the wrong name

Start with the most useful thing almost no one is told: the panel is misnamed. The common enzyme tests — ALT, AST, ALP and GGT — don't measure the liver's function. They reflect either irritation and injury to liver cells, or a problem with the bile ducts. The only numbers on the panel that genuinely reflect the liver's synthetic function — the real work it does, like making proteins and clearing waste — are albumin and bilirubin, together with your clotting (the INR) ¹⁴.

That distinction matters because it changes how you read a flagged result. A raised ALT doesn't mean your liver has "stopped working." It means some liver cells have leaked an enzyme into the blood — a sign of irritation, which has many causes, most of them not catastrophic.

What ALT and AST actually mean

ALT (alanine aminotransferase) and AST (aspartate aminotransferase) are the two you'll see most. Both are enzymes that live inside liver cells, so when those cells are damaged or irritated they spill out into the bloodstream and the levels on your report go up. They are markers of hepatocellular injury — that is, injury to the liver cells themselves ¹.

The key difference is specificity. ALT is the more liver-specific of the two — it's found mainly in the liver, so a raised ALT points fairly squarely there. AST is less specific: it's also present in muscle, the heart and red blood cells, so an isolated raised AST can come from sources that have nothing to do with the liver at all ¹. That single fact resolves a lot of needless worry, as we'll come back to in the limits below.

What GGT and ALP are telling you

The other two enzymes point at a different part of the system: the bile ducts, the plumbing that drains bile out of the liver. When that drainage is obstructed or irritated — a state called cholestasis — ALP (alkaline phosphatase) and GGT (gamma-glutamyl transferase) tend to rise ¹.

GGT has two practical uses worth knowing. First, it's often raised by alcohol, so it can be a useful (if non-specific) signal there. Second, it helps with a common puzzle: ALP isn't only made in the liver — it's also made in bone — so when ALP is up, a raised GGT alongside it confirms the source is the liver rather than bone ¹. It's the test that tells two innocent-looking elevations apart.

The most common reason for a mildly raised ALT

For most reasonably well people who get a surprise flag, the answer is fatty liver — non-alcoholic fatty liver disease (NAFLD), increasingly renamed MASLD (metabolic dysfunction-associated steatotic liver disease). It is the most common cause of a mildly raised ALT, and it has a recognisable signature: ALT and AST raised only modestly — typically around 2–3 times the upper limit of normal — usually with an AST:ALT ratio under 1. (A ratio above 2 points more toward alcohol-related liver injury instead) ¹.

What makes fatty liver so relevant to the people we see is what drives it. It's a metabolic condition, strongly tied to excess weight, type 2 diabetes and high triglycerides — and it very often causes no symptoms at all ¹³. That's why it so often shows up as nothing more than an unexpected enzyme on a routine panel. It's also why your liver numbers are best read alongside your blood-sugar results and your cholesterol and triglyceride panel — the same metabolic picture sits behind all three. The encouraging part: caught early, fatty liver is frequently reversible with changes to weight, diet and activity.

What these tests can't tell you

This is where honesty matters most, because the panel has real limits.

Normal enzymes do not rule out serious disease. This is the big one. Enzyme levels can be entirely normal even in significant or advanced liver disease, including cirrhosis ¹. A reassuring panel is genuinely reassuring — but it is not a clean bill of health for your liver.

The numbers are non-specific. As noted, AST in particular rises from causes that aren't the liver at all — muscle injury, strenuous exercise, even heart problems can push it up ¹. A raised AST after a hard week at the gym is a familiar false alarm.

The size of the rise doesn't track severity. It's tempting to read a higher number as worse disease, but the degree of enzyme elevation does not reliably reflect how severe or how serious the underlying problem is ¹. When doctors actually need to grade liver disease and prognosis, they use scores like Child-Pugh and MELD — built from bilirubin, albumin and INR, the true-function markers — not from ALT or AST ¹. The enzymes flag that something is happening; the function tests measure how much it matters.

One more practical caveat: the upper limit of normal varies by laboratory, assay, sex and age ¹. A value flagged at one lab might sit inside the range at another, which is one reason a single out-of-range enzyme is read in context rather than as a verdict.

What we see at the clinic

A raised ALT is one of the most common surprises on the baseline panels we run in Pattaya, and the first thing we usually do is take the worry out of the room. In most of the expats and medical travellers we see, a mild elevation traces back to the metabolic picture — weight, blood sugar and triglycerides drifting together — and, for some, alcohol, which we raise as one common and very reversible contributor rather than a judgement. We don't treat liver disease, and we don't diagnose it from a single enzyme; where a number or a pattern warrants it, you'll see a doctor and, if needed, a specialist. What we do is establish an honest baseline, read the liver numbers alongside the rest of the metabolic panel, and explain what they mean — particularly the two facts people are rarely told: that fatty liver is common and often reversible early, and that normal enzymes don't fully exclude a problem. Fatty liver tracks closely with metabolic disease in Thai populations, so it's a pattern we see regularly ⁵.

Common questions

My ALT is a little high — should I panic? Almost certainly not. A mild rise — often around 2–3 times the upper limit — is most commonly fatty liver, a metabolic and frequently reversible condition, not a sign of liver failure ¹³. A doctor will read it alongside your other numbers and usually repeat the test rather than jump to conclusions.

If my liver enzymes are normal, is my liver definitely fine? Not definitely. It's genuinely reassuring, but enzyme levels can be normal even in significant liver disease, including cirrhosis ¹. That's exactly why results are interpreted in the context of your history rather than as a single all-clear.

Does a raised AST mean something different from a raised ALT? It can. ALT is more specific to the liver, while AST also comes from muscle, the heart and red blood cells — so an isolated raised AST may have nothing to do with your liver, sometimes just hard exercise ¹. The pattern between them, including the AST:ALT ratio, is part of what a doctor reads.

My GGT is up — is that about alcohol? It can be, since GGT often rises with alcohol — but it's non-specific, so a raised GGT is not proof of anything on its own ¹. It's also used more technically to confirm that a raised ALP is coming from the liver rather than from bone.

Can fatty liver really be reversed? Often, yes — especially when it's caught early, before lasting scarring. Because it's driven by weight, blood sugar and triglycerides, the same changes that improve those tend to improve the liver too ³. That's a conversation to have with your doctor, who can tailor it to you.

Key takeaway

The name "liver function tests" oversells what most of the panel does. ALT, AST, ALP and GGT are injury and bile-duct markers, not measures of how well your liver works; only albumin, bilirubin and your clotting (INR) reflect its true function ¹⁴. A mildly raised ALT is most often fatty liver — common, usually silent, tied to your metabolic health, and frequently reversible when caught early ¹³. Just as importantly, normal enzymes don't fully exclude serious disease, and the size of a rise doesn't measure its severity ¹. The sensible move isn't alarm at a single red flag — it's an honest baseline, read in context and against your own trend over time, with a doctor joining the dots.