Biomarkers · Vitamins & Endocrine

Free Triiodothyronine (Free T3)

Higher values go with lower risk of death. Not measured in our NHANES panel. The curve is drawn from the published study (source below).

Free T3 and risk of death

Published curve higher than the NHANES fitNHANES fit higher

Risk of death relative to the curve's reference value, drawn from the published study (source below).

Published risk of death0.81×per SD higher free triiodothyronine (FT3) (95% CI 0.70–0.93) · NHANES 2007-2012 U.S. participants with measured thyroid function and no prior thyroid disease; NDI follow-up to Dec 31, 2015

Free T3 by age and sex

Each bar runs from the 5th to the 95th percentile, the darker part from the 25th to the 75th, and the tick is the median. Not a reference range or a target.

The percentiles as a table

Each row is one sex and age band: the values below which 5%, 25%, 50%, 75% and 95% of people fall (pg/mL), from the survey named in the row. Not a reference range or a target.
Sex	Age	5th	25th	Median	75th	95th	n	Cycle / source
All	20-39	2.74	3.06	3.26	3.52	3.96	570	NHANES 2011-2012
All	40-59	2.67	2.91	3.13	3.32	3.69	508	NHANES 2011-2012
All	60+	2.43	2.73	2.92	3.17	3.57	512	NHANES 2011-2012
All	all	2.58	2.90	3.13	3.36	3.80	1,590	NHANES 2011-2012
Men	20-39	2.91	3.18	3.38	3.63	4.05	295	NHANES 2011-2012
Men	40-59	2.78	3.09	3.26	3.44	3.80	252	NHANES 2011-2012
Men	60+	2.48	2.72	2.90	3.17	3.41	260	NHANES 2011-2012
Men	all	2.66	3.01	3.23	3.45	3.90	807	NHANES 2011-2012
Women	20-39	2.66	2.99	3.16	3.38	3.82	275	NHANES 2011-2012
Women	40-59	2.64	2.80	3.00	3.18	3.62	256	NHANES 2011-2012
Women	60+	2.36	2.73	2.93	3.18	3.62	252	NHANES 2011-2012
Women	all	2.54	2.83	3.05	3.24	3.67	783	NHANES 2011-2012

What studies found when this marker was changed

They describe what happened in those study groups, not a recommendation. See all catalogued interventions.

Measured changes in Free T3, study by study

1 effect with a numeric size, each from one study arm: the dot is the reported change, the line its 95% interval where one was given. Green points move the marker the way lower risk of death lies; amber the other way. Study results in the groups studied, not predictions for a person.

Each row is one study: what was changed, the effect it reported on this marker, and the source.
Type	Intervention	Reported effect	Evidence	Source
diet	Gluten-free diet vs control diet in adults with non-coeliac Hashimoto's thyroiditis (3 RCTs)	Free T3 changed -0.18 pg/mL (95% CI -0.50 to 0.14) with a gluten-free diet vs control; 3 RCTs, n=110; duration not stated	meta-analysis	Araújo EMQ, et al. Nutrients. 2025;17(21):3437.

Things that worsen it

Each row is one study: what was changed, the effect it reported on this marker, and the source.
Type	Condition or exposure	Reported effect	Evidence	Source
pharmacologic	Levothyroxine given once weekly at 7 times the usual daily dose, compared with daily dosing, in hypothyroid adults (randomized crossover)	Trough free T3 208 pg/dL with weekly vs 242 pg/dL with daily dosing (P < 0.01); TSH higher (6.61 vs 3.92 microIU/mL)	RCT	Grebe SK, et al. The Journal of clinical endocrinology and metabolism. 1997…

What else moves this marker

They describe those study groups, not an individual, and an association is not a cause.

Diseases and conditions

Each row is one study: what it compared and the change it reported in this marker.
What	Effect on the marker	Comparison · population	Source
↓ Iron deficiency · patients with iron deficiency vs healthy controls without iron deficiency	-0.143 pg/mL (95% CI -0.208 to -0.078)	iron deficiency vs no iron deficiency · 10 cross-sectional studies; pregnant and non-pregnant women subgroups	meta analysis · PMID 38004184

Genetic predisposition

Each row is one study: what it compared and the change it reported in this marker.
What	Effect on the marker	Comparison · population	Source
↕ Genome-wide significant loci for free T3 (ThyroidOmics) · total variation in free T3 concentration explained by associated loci	+1.1%	variance explained · European-ancestry adults with reference-range thyroid function	gwas · PMID 38291025

Sources

Risk curve: Lang X 2022 (PMID 36060933): all-cause HR 0.81 (0.7-0.93) per SD higher free triiodothyronine (FT3). Scaled to US values (NHANES).
Lang X, Li Y, Zhang D, Zhang Y, Wu N, Zhang Y. FT3/FT4 ratio is correlated with all-cause mortality, cardiovascular mortality, and cardiovascular disease risk: NHANES 2007-2012. Frontiers in Endocrinology. 2022;13:964822.

Map Free Triiodothyronine (Free T3) against another biomarker.

Map Free T3 against another marker