Free T4 (Thyroxine)

Main thyroid hormone produced by thyroid gland

16/22Providers
Thyroid FunctionCategory
weight gainReference
Widely Available

16 of 22 providers

Category

Thyroid Function

Reference Range

weight gain

What is Free T4 (Thyroxine)?

Free T4 (thyroxine) is the unbound, biologically active form of the main thyroid hormone circulating in your blood. Your thyroid gland produces mostly T4 (about 80-90% of thyroid hormone output), which then gets converted to the more potent T3 in your tissues. Only about 0.03% of total T4 is "free"(unbound to proteins like thyroid-binding globulin)—the rest is bound and inactive.

Here's the key insight:T4 is essentially a "prohormone"—it's not the final active form. Think of T4 as the raw material that your body converts into the 3-4x more potent T3 hormone where it's needed (liver, kidneys, muscles, brain). About 80% of circulating T3 comes from peripheral conversion of T4 to T3, not from direct thyroid secretion. This conversion process depends on enzymes called deiodinases, which require selenium, zinc, and iron to function properly.

This is why some people have normal TSH and normal Free T4 but still have hypothyroid symptoms—if they're not efficiently converting T4 to T3 (due to stress, nutrient deficiencies, medications, or chronic illness), they won't have enough of the active hormone. That's why checking Free T3 in addition to TSH and Free T4 is essential for a complete thyroid assessment.

Interpreting TSH + Free T4 Together

  • High TSH + Low Free T4:Primary hypothyroidism (thyroid gland failure)—most common pattern. Start levothyroxine.
  • High TSH + Normal Free T4:Subclinical hypothyroidism (early thyroid dysfunction). Monitor or treat depending on symptoms/antibodies.
  • Low TSH + High Free T4:Primary hyperthyroidism (overactive thyroid). Evaluate for Graves'disease or toxic nodule.
  • Low TSH + Low/Normal Free T4:Central hypothyroidism (pituitary or hypothalamus problem)—rare but serious. Refer to endocrinology.
  • Normal TSH + Low Free T4:Rare;may indicate central hypothyroidism or assay interference. Retest and check Free T3.
  • Normal TSH + Normal Free T4 but symptoms:Check Free T3—may have T4→T3 conversion problem. Also consider reverse T3.

Optimal vs Standard Ranges

Optimal (Longevity)
1.0-1.5 ng/dL▼
  • Mid-upper normal range;best metabolic function and symptom control
  • Many functional medicine doctors target this range
Low-Normal
0.8-1.0 ng/dL▼
  • Within standard range but may have subtle hypothyroid symptoms
  • Consider increasing levothyroxine dose if symptomatic
Low (Hypothyroid)
<0.8 ng/dL▼
  • Hypothyroidism;insufficient thyroid hormone production
  • Requires treatment with levothyroxine
  • Check TSH to confirm
High (Hyperthyroid)
>1.8 ng/dL▼
  • Hyperthyroidism;excess thyroid hormone
  • Causes rapid heart rate, anxiety, weight loss
  • Check TSH (will be low)
  • Evaluate for Graves'disease
Standard lab range: weight gain
Sources:PMID:37696273, PMID:24195179, PMID:20858880

Scientific Evidence

Free T4 vs Total T4

Only ~0.03% of T4 is "free"(unbound). The rest is bound to thyroid-binding globulin (TBG), albumin, and transthyretin. Bound T4 is inactive. Free T4 reflects biologically active hormone and is preferred over Total T4, which fluctuates with binding protein levels (pregnancy, estrogen, liver disease).

Source:Thienpont LM, et al. Standardization of free thyroxine measurements:critical considerations. Eur Thyroid J. 2015;4(Suppl 1):51-58. (PubMed)

T4 as Prohormone

T4 has relatively low intrinsic activity. It's converted to T3 by deiodinases in peripheral tissues (liver, kidney, muscle, brain). About 80% of circulating T3 comes from T4→T3 conversion, not direct thyroid secretion. T3 is 3-4x more potent than T4 at binding thyroid receptors. This is why some patients with normal Free T4 still have symptoms if conversion is impaired.

Source:Bianco AC, Kim BW. Deiodinases:implications of the local control of thyroid hormone action. J Clin Invest. 2006;116(10):2571-2579. (PubMed)

Levothyroxine Treatment Targets

Standard treatment goal is to normalize TSH (0.5-2.5 mIU/L) and bring Free T4 into mid-upper normal range (1.0-1.5 ng/dL). However, 10-20% of patients on levothyroxine report persistent symptoms despite normal TSH/Free T4, often due to low Free T3. Adding T3 (liothyronine) may benefit this subset.

Source:Jonklaas J, et al. Guidelines for the treatment of hypothyroidism. Thyroid. 2014;24(12):1670-1751.

Selenium and Thyroid Function

Deiodinase enzymes that convert T4→T3 are selenoproteins requiring selenium. Selenium deficiency impairs T3 production and increases reverse T3. Selenium supplementation (200 mcg/day) improves thyroid hormone metabolism and reduces TPO antibodies in Hashimoto's patients by ~40%.

Source:Ventura M, et al. Selenium and thyroid disease:from pathophysiology to treatment. Int J Endocrinol. 2017;2017:1297658.

Central Hypothyroidism

Low Free T4 with low or inappropriately normal TSH indicates pituitary or hypothalamic dysfunction, not primary thyroid failure. Prevalence is ~1:20,000-80,000. Often associated with other pituitary hormone deficiencies (GH, ACTH, LH/FSH). Requires pituitary MRI and endocrine workup.

Source:Persani L, et al. Central hypothyroidism. J Clin Endocrinol Metab. 2018;103(11):4126-4136.

Which Providers Test Free T4 (Thyroxine)?

Superpower
Included in panel
Blueprint
Included in panel
Mito Health
Included in panel
Function
Included in panel
Marek Comp.
Included in panel
Marek Complete
Included in panel
Marek Exec.
Included in panel
Life Ext.
Included in panel
Labcorp
Included in panel
Everlywell
Included in panel
Empirical Health
Included in panel
SiPhox Health
Included in panel
Healthspan
Included in panel
Hims Adv.
Included in panel
Vitality Std
Included in panel
Vitality Elite
Included in panel

Full Provider Comparison

ProviderIncludesAnnual CostBiomarkers
SuperpowerSuperpower✓$199100+ (150 with ratios)
WHOOP Advanced LabsWHOOP Advanced Labs—$34965
Labcorp OnDemandLabcorp OnDemand✓$39830+
Life ExtensionLife Extension✓$48640+
EverlywellEverlywell✓$444288
Mito HealthMito Health✓$349100+
InsideTrackerInsideTracker—$76154
Function HealthFunction Health✓$365160+
Marek Health BaseMarek Health Base—$25065
Marek Health ComprehensiveMarek Health Comprehensive✓$49570+
Marek Health CompleteMarek Health Complete✓$895100+
Marek Health ExecutiveMarek Health Executive✓$1950150+
BlueprintBlueprint✓$37580+
Quest HealthQuest Health—$Varies75+
Empirical HealthEmpirical Health✓$190100+
Oura Health PanelsOura Health Panels—$9950
SiPhox HealthSiPhox Health✓$12460
Hims Labs BaseHims Labs Base—$19950
Hims Labs AdvancedHims Labs Advanced✓$499120+
HealthspanHealthspan✓$418870-80+
Vitality Blueprint StandardVitality Blueprint Standard✓$37585
Vitality Blueprint EliteVitality Blueprint Elite✓$700128

Related Biomarkers

Other biomarkers in the Thyroid Function category:

TSH (Thyroid Stimulating Hormone) Free T3 (Triiodothyronine) TPO Antibodies (Thyroid Peroxidase) Thyroglobulin Antibodies Iodine Selenium T3 Uptake Total T4

Frequently Asked Questions

What is Free T4 (Thyroxine)?

Main thyroid hormone produced by thyroid gland

What is the optimal range for Free T4 (Thyroxine)?

The standard reference range for Free T4 (Thyroxine) is weight gain. Optimal ranges may differ based on individual health goals and expert recommendations.

Which blood test providers include Free T4 (Thyroxine)?

16 out of 22 blood testing providers include Free T4 (Thyroxine) in their panels. This biomarker is widely available across major providers.

What category does Free T4 (Thyroxine) fall under?

Free T4 (Thyroxine) is categorized under Thyroid Function. This category includes biomarkers that help assess related aspects of health and wellness.

Medical Disclaimer

This information is for educational purposes only and is not medical advice. Always consult with a qualified healthcare provider about your specific health needs.

Last reviewed:2026-02-20