SHBG: the lab value that explains why your hormones look normal

You are sitting at the kitchen table with a sheet of paper. Your name at the top, below it a column of values and a column of reference ranges. Nothing is printed in bold. Nothing has an asterisk. Everything is within range.

And still something is off. The hairs on your chin are new. Your cycle comes when it feels like it. Your skin behaves as it did at seventeen. Your energy lasts until four in the afternoon and then it is over.

Many women know this pattern. The report says fine, the body says something else, and nobody can explain both at the same time. This is not a problem of imagination. It is usually a problem of translation.

Because a hormone value in the blood tells you how much hormone is on the move. It does not tell you how much of it arrives. Between those two sentences sits a protein with an unwieldy name: sex hormone binding globulin, SHBG for short.

That is why I measure SHBG alongside, before I tick off a hormone result as unremarkable. And that is why, with a low value, I look first at the liver, the insulin situation and the thyroid, before I talk about hormones.

One sentence up front: the gynaecological workup remains the foundation. It answers questions that no lab value can answer. The metabolic perspective comes on top of it, not in its place. That it is not routinely part of basic diagnostics has to do with the purpose of that diagnostic work and not with the care of those who carry it out.

These seven labels sit next to every study. Between an effect in cell culture and an effect in a thousand women lie worlds, and on the internet both regularly end up in the same pot.

Why two identical lab values can mean two different women

Picture two women, same age, same report. Both have the same total testosterone, somewhere in the upper half of the reference range, let us say. On paper they are twins.

One of them has no complaints at all. The other struggles with hair growth on her chin, with oily skin, with a cycle that keeps shifting. How can that be, when the number is the same?

Because the number measures the wrong thing. More precisely: it measures everything, and only a small part of it is interesting.

The image with the crates of bottles

I like this image better than the dimmer switch you often read about. Imagine your blood is a drinks store. The report says how many bottles are in the shop. That is total testosterone.

But most bottles are in crates, strapped down, on pallets. They are there, they count, they are not within reach for the moment. Only the few bottles standing loose on the shelf can be picked up and opened.

SHBG is the pallet. It packs testosterone and estradiol up and carries them through the blood. Packed hormone travels along without arriving. Only the free fraction can dock onto the receptor of a cell and set something off there.

And now back to the two women. One has a lot of SHBG. In her, almost all the bottles are in crates. The other has little SHBG. In her, a much larger share stands open on the shelf. Same total number, completely different availability.

By the way, this is not only about testosterone. SHBG also binds estradiol, although with less force. That means the same arithmetic applies to the estrogen someone read out to you as unremarkable.

Which test makes sense when is covered in hormone testing in women. Here the takeaway is enough: SHBG is the value that makes the others readable in the first place.

And now you know why two identical numbers can tell two different stories.

What SHBG actually is and how much hormone is really free

A short inventory, so the word stops sounding like exam material.

SHBG is a protein. It is built in the liver and released into the blood. There it binds sex steroids, meaning testosterone, dihydrotestosterone and estradiol. It does not do this indiscriminately but with different force: it holds testosterone more tightly than estradiol.

Besides SHBG there is a second binding partner: albumin. It is the most common protein in the blood, an all rounder that binds almost everything a little. Albumin holds hormones considerably more loosely than SHBG. That is why hormone bound to albumin is often counted towards the so called bioavailable fraction.

Let us stay with the drinks store. SHBG is the strapped down pallets. Albumin is the crates standing open in the aisle, from which you can pull a bottle with some effort. And the small remainder really does stand free.

The honest limitation that appears almost nowhere

Now comes the part most pages leave out, because it disturbs the neat explanation. The free hormone hypothesis is a model. Not a law of nature.

Why am I telling you this while at the same time making a case for the value? Because the two belong together. A value whose limits you know is worth more to you than a value you take for a final verdict.

A side note that shows how young this field still is: according to the paper from Vancouver, SHBG also accumulates outside the vessels, where it may unfold effects of its own (Hammond 2011, PMID: 21613632). Basic research, not an instruction for action.

And now you know why I am enthusiastic and cautious about this value at the same time.

What pulls SHBG down, insulin above all

If your SHBG is low, the first sensible question is not how to get it up. It is: why is it down?

And there is one factor that towers above all the others. Insulin.

This is not a side note from functional medicine. It is one of the few chains in this field that runs cleanly through three levels of evidence. Cell culture, animal model, human. I will show you all three, and with each one I will tell you how far it carries.

Level one: the liver cell in the test tube

Level two: why it happens

Almost twenty years later the same line of research found the switch that gets turned. And the answer is surprisingly precise.

How the liver ends up in that state is covered in sugar, fructose and the liver. One sentence is enough here: the liver is the building site of SHBG.

Level three: the human

Now comes the study I consider the most elegant in this whole field. It is tiny, it is old, it has no control group, and it still goes at exactly the question at hand.

And now the finding that keeps the story from becoming too smooth. The same group carried out the same intervention in five slim, healthy women with a regular menstrual cycle.

What else pushes it down

Insulin is the strongest single factor, but not the only one. The following points appear in every endocrinology textbook and in every German lab catalogue. For some of them I found no primary study with robust numbers in this research, and therefore I name them without numbers and mark them as textbook knowledge.

What insulin resistance is and what it does to the menstrual cycle is covered in insulin resistance and hormones in women. Here I stay with the one question: what does it do to the transport protein.

And now you know why, with a low SHBG, I talk about the liver and blood sugar first and not about hormones.

What pushes SHBG up, estrogens and the pill above all

There is one situation in which an SHBG value is regularly misunderstood. And in both directions.

The value is high, very high even, and someone reads that as good news. Or the value is high and someone reads it as illness. Both may be off the mark when one decisive circumstance is not on the list: the use of an oral estrogen.

The pill does not change things a little, it changes the order of magnitude

Why does ethinylestradiol do this? Because it runs through the liver. A hormone that is swallowed passes the liver before it reaches the rest of the circulation. And there it meets exactly the cells that build SHBG. They respond by ramping production up.

The thyroid, the second big lever upwards

SHBG responds to thyroid hormone. And so clearly that an undetected thyroid problem may throw off every androgen calculation.

Which thyroid values actually count and which can be skipped is covered in thyroid blood values: which ones really count. And how the thyroid and the menstrual cycle hang together is in the thyroid and female hormones.

Age, pregnancy, liver

In men SHBG rises over the years. That has a practical consequence which is rarely spelled out.

In pregnancy SHBG rises strongly. That is physiological and not a red flag. It does have one methodological consequence that matters in a moment: the calculation procedures for free testosterone do not apply in pregnancy.

And now you know why the first question with a striking SHBG is what is currently being taken.

Why a low SHBG may be an early pointer to the metabolic situation

Now comes the part that turns SHBG from a side value into something interesting. And the part where I have to be most careful, so that an association does not become a prophecy.

Blood sugar is a late value. It becomes noticeable when the pancreas can no longer compensate. Years before that a quiet phase runs, in which the body pushes back with more and more insulin and sugar therefore still looks normal.

And SHBG? It responds to exactly that insulin. Not to the sugar. That makes it an indirect window into a phase that is otherwise hard to see.

The obvious objection: perhaps the low SHBG is only a side effect. People with more abdominal fat often have more insulin, more insulin may lower SHBG, and the same people more often develop diabetes. The value would then be a spectator and not a player.

The same research group attacked this objection three years later with a clever trick.

What the value reflects in large groups

Where that point of leverage lies in practice is covered in insulin resistance and weight loss and in silent inflammation and weight.

One more thing, explicitly: SHBG is not a diagnostic criterion for polycystic ovary syndrome. The diagnosis follows its own internationally agreed criteria and belongs in medical hands. More on that in understanding polycystic ovary syndrome.

And now you know why I consider this value a good early signal and still send nobody home with it alone.

Free testosterone, the free androgen index and the limits of the calculation

If you search the internet for free testosterone, you land on a calculator within seconds. Number in, number out, done. I want to explain why I am not giving you a formula here, and why that is not paternalism.

There are three routes, and they are not equally good

The first route is measurement in the lab via an elaborate separation procedure, equilibrium dialysis. That is the reference method. It is expensive and rarely done.

The second route is calculation from total testosterone, SHBG and albumin via a binding model. Online calculators and lab software exist for this.

The third route is the free androgen index, FAI for short. It simply puts total testosterone in relation to SHBG. It is no more calculation than that, and precisely there lies its problem: albumin does not appear in it.

What the guidelines say about it

Here is the point where my view and the state of the guidelines explicitly coincide. That matters to me, because with integrative medicine the suspicion of a fringe opinion is quickly in the room.

Why there is no formula here

I could write a formula out for you now. I am not doing it, for three reasons.

First, a number without context changes nothing about your situation but may create fear. Second, the results of different calculators and lab methods are not readily comparable. And third: no treatment follows from a calculated value. A conversation follows.

If you have a testosterone value in front of you and want to place it, the detailed reading guide is in understanding testosterone values. That article covers time of day, assay and repeat measurement.

And now you know why the quick calculation on the internet is not the answer you are looking for.

The practical part: when and how SHBG is sensibly measured

So far there has been a lot of physiology. Now the part you can take with you into your next consultation.

When the value belongs in the panel

Cycle day, time of day, fasting

Here I want to be more cautious than the internet usually is on this point.

SHBG is considerably more stable across the menstrual cycle than estradiol and progesterone, which can multiply within a few days. I did not find a good, clearly citable paper in this research showing a relevant SHBG fluctuation across the cycle. So I give you no number and no SHBG specific ideal day.

In practice that means: the cycle day of the blood draw follows the hormones measured alongside. For progesterone that is the second half of the cycle, for estradiol it depends on the question. SHBG simply rides along.

On fasting, the same principle. For SHBG itself I found no robust primary source. Blood is drawn fasting because of the other values: guidelines place testosterone in a fasting morning sample, and glucose and insulin need it anyway.

On the pill: one value, two different questions

What a low value means in a woman and what it means in a man

Why this article contains no reference table

That is a deliberate decision, and you should know the reason.

SHBG is measured with different immunoassays that are not standardised against each other. Every lab sets its reference range using its own method and its own comparison group. On top of that, the value diverges widely by sex, age, cycle phase, pregnancy and hormone use.

A table on the internet would therefore fake a universally valid boundary. That boundary does not exist. What applies is the range printed next to the value on your own report, and the most informative thing is the trend in the same lab.

And now you know which question you can ask at your next appointment: does my total value fit my SHBG, and what does that mean for the free fraction?

What actually shifted SHBG in studies

Finally the most honest part. Not what could theoretically move something, but what actually shifted numbers in controlled investigations. With effect sizes, and with what moved nothing.

What cannot be shifted

And now the part that protects against self reproach. Part of your SHBG value is predisposition.

The same gene locus was also found in 3,495 Chinese men (Chen 2013, PMID: 24049095 · DOI: 10.1136/jmedgenet-2013-101705) [Cohort, GWAS].

And where does the environment come in?

Now comes the section I have to explain most often in my practice. Because an idea circulates on the internet that sounds logical and that the data do not support.

The idea goes like this: xenoestrogens, meaning hormone like environmental substances, displace your own hormones from the transport protein and thereby set them free. Sounds plausible. It was investigated. The available data do not support it that way.

Why I go into this at such length, although it runs counter to my own direction of enquiry: because in consultations I ask about mould, endocrine disruptors and heavy metals, and because that question is only worth something if it is asked in the right place. Not at the transport protein, but rather where the available data point: liver and insulin situation. Silent inflammation I add as a working hypothesis, for SHBG that route is not documented.

Where endocrine disruptors actually occur in everyday life and what is documented about them is covered in xenoestrogens and endocrine disruptors in everyday life. And how the liver breaks estrogen down is in lowering estrogen naturally: how the liver breaks estrogen down.

A word against the avoidance spiral

I have seen what happens when environmental topics are read without a brake. A sensible question turns into constant monitoring. Caution turns into control. Control turns into an everyday life that keeps narrowing, in which more and more foods, products and situations drop away.

That is not a good path. Chronic stress and heavily restricted eating are themselves strong influences on exactly this axis. When the preoccupation with values and avoidance becomes narrow, that is a topic in its own right. Understanding eating disorders: body and mind describes where that line runs.

My rule of thumb for environmental topics: change the three things that are biggest and easiest in your own everyday life, and then stop searching. The rest is a matter for an examination, not for research on the internet.

And now you know what actually moved in studies, and what is predisposition.

Common questions about SHBG

Where SHBG connects to the rest of the system

SHBG does not stand on its own. It hangs on the liver, on insulin, on the thyroid, on contraception and on the question of how a person deals with an unclear result. These articles carry the threads further.