# Transferrin saturation and free testosterone (TSAT)

Two indices VT computes only when a clinician has ordered their inputs: the share of your iron-carrying protein that is actually carrying iron, and the fraction of your testosterone that is not bound to the protein that holds most of it.

Type: index · Unit: % (saturation), nmol/L or pmol/L (free testosterone) · Domain: Nutrition & Energy Balance · Accuracy: Calculated · Importance: Context

Also called: iron saturation, iron studies, TIBC, free testosterone, calculated free testosterone, Vermeulen, bioavailable testosterone, SHBG, cFT, FT

## In one sentence

Two indices VT computes only when a clinician has ordered their inputs: the share of your iron-carrying protein that is actually carrying iron, and the fraction of your testosterone that is not bound to the protein that holds most of it.

## Why it matters

Transferrin saturation is iron divided by the total iron-binding capacity, and it reads the iron in transit rather than the iron in store, which ferritin reads; the two together separate an empty store from a store the body is holding back during inflammation, the distinction the iron-deficiency literature turns on (Camaschella 2015). The clinician specification's working reading is 20 to 45 percent, with iron deficiency without anemia considered when ferritin is below 30 µg/L, or below 100 with a raised CRP, both as the specification's working rule. Free testosterone exists because most testosterone travels bound to sex hormone-binding globulin and albumin, and the total can mislead when the binding protein is high or low: in a large series of sera with binding capacities from very low to very high, the free fraction calculated from total testosterone, the binding globulin and albumin was almost identical to the value measured by equilibrium dialysis in every condition except pregnancy, while the direct analog immunoassays and the free androgen index were not reliable (Vermeulen 1999). The Endocrine Society's guideline asks for a fasting morning total testosterone, repeated to confirm, and a free testosterone by dialysis or an accurate formula when the total is near the lower limit or the binding globulin is altered (Bhasin 2018); the harmonized reference range for healthy non-obese men aged 19 to 39, measured against a reference method across four cohorts, ran from 264 to 916 ng/dL, about 9.2 to 31.8 nmol/L (Travison 2017).

## What moves it

Serum iron, and so the saturation, swings through the day (highest in the morning) and jumps for a day after an iron tablet or a meat meal; inflammation lowers it as the body sequesters iron. Testosterone is highest in the morning, falls after a short night, a hard training block, an illness or weight gain, and the binding globulin rises with age, thyroid hormone, estrogen and a low-carbohydrate diet and falls with insulin resistance and body fat, which is the whole reason the free fraction is computed.

## How to measure it well

Both need a fasting morning draw: iron studies with no iron supplement for 24 hours and no draw within two weeks of being unwell; testosterone in the morning, fasting, repeated on a second morning before anything is concluded, with the binding globulin and albumin on the same draw. Neither is on VT's panel; both are a clinician's decision.

## How Vitality Tracker uses it

Computed only when every input comes from one collection: transferrin saturation from iron and TIBC (or from iron and transferrin where the laboratory reports that), free testosterone by the Vermeulen method from total testosterone, SHBG and albumin. Each is shown in its own panel (iron; hormones) with the laboratory's range and flag first, never overridden, and the working figures beneath in smaller type marked *working*: saturation 20 to 45 percent; total testosterone 15 nmol/L and above in men and free testosterone 0.25 nmol/L and above, both v0.1 and both on the clinical advisor's list. Women's hormone panels are cycle-phased and are a clinician's design; VT stores them as reported and computes nothing from them.

## How far to trust the number

Calculated from laboratory results with large day-to-day and time-of-day swings; the morning rule and the repeat are the measurement. The Vermeulen calculation assumes standard binding constants and fails in pregnancy.

## Method

TSAT (%) = serum iron (µmol/L) ÷ TIBC (µmol/L) × 100. Free testosterone (Vermeulen): solve the mass-action equation for free T given total T, SHBG and albumin with the published association constants for SHBG and albumin; VT uses the constants from the paper. Conversions: testosterone ng/dL × 0.0347 = nmol/L; free testosterone pg/mL × 3.47 = pmol/L.

## Sources

1. Camaschella C. N Engl J Med 2015;372:1832–43. [doi:10.1056/NEJMra1401038](https://doi.org/10.1056/NEJMra1401038)
2. Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab 1999;84:3666–72. [doi:10.1210/jcem.84.10.6079](https://doi.org/10.1210/jcem.84.10.6079)
3. Bhasin S, et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab 2018;103:1715–44. [doi:10.1210/jc.2018-00229](https://doi.org/10.1210/jc.2018-00229)
4. Travison TG, et al. Harmonized reference ranges for circulating testosterone levels in men of four cohort studies in the United States and Europe. J Clin Endocrinol Metab 2017;102:1161–73. [doi:10.1210/jc.2016-2935](https://doi.org/10.1210/jc.2016-2935)
5. Vitality Tracker, Clinician report content specification (derived indices); optimal set v0.1 (working).

## Related

- [Ferritin](https://vitalitytracker.app/knowledge/ferritin)
- [CRP, high-sensitivity](https://vitalitytracker.app/knowledge/c-reactive-protein)
- [Full blood count with differential](https://vitalitytracker.app/knowledge/full-blood-count)
- [Getting your blood tests done in New Zealand](https://vitalitytracker.app/knowledge/blood-tests-in-new-zealand)

Reviewed October 2026 · v1

Source: https://vitalitytracker.app/knowledge/transferrin-saturation-and-free-testosterone