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Lexicon
Weight management

T3 (Liothyronine)

The active thyroid hormone - drives metabolism directly, to be handled with respect.

1 min read 10 sources

To the product To the sources
White tablet bottle with the black TPD label for T3 on a white podium, next to a blue 3D molecule model with purple iodine atoms in a bright lab
Compound approved · in the USA since 1956Lab tested in the shop

Summary

At a glance

Sources checked on 26 Oct 2026

Half-life in the blood
~22 hours

after a single dose in 12 healthy people, longer depending on the time points3

Potency compared with T4
~3 -fold

same TSH suppression with about a third of the amount (ratio 0.36)2

LDL cholesterol
-13.3 %

T3 instead of T4 at the same TSH, 14 people with hypothyroidism1

Until peak level
2.5 hours

T3 in the blood after a single dose, heart rate briefly raised after about 5 hours3

  • Liothyronine is the synthetic form of T3, the active thyroid hormone. It binds directly to thyroid hormone receptors without the detour via T4.810
  • About a third of the amount of T4 is enough for the same TSH suppression. After a single dose T3 peaks in the blood after 2.5 hours, the half-life was about 22 hours.23
  • Compared with T4 at the same TSH, T3 lowered body weight, total cholesterol, LDL and ApoB. Heart rate, blood pressure and insulin sensitivity did not differ.1
  • As an add-on to an antidepressant T3 brought no benefit in a study of 153 people.5
  • In the USA liothyronine has been approved as a prescription tablet since 1956. Everything here is the state of research, not a recommendation.6
Profile

Updated June 2026

How long does it stay in the body?

Share left after one dose, calculated with a half-life of ~24 hours

Half-life ~24 hours
After 48 hours: 25 %
Hour 24 · 50 %Hour 48 · 25 %Hour 72 · 12.5 %Hour 120 · 3.1 %

Simplified model without an absorption phase, half-life from the TPD lexicon.

Class
Thyroid hormone

active T3

Use
Oral

1-2x per day

Half-life
about 1 day
Status
Clinically approved

hypothyroidism

Sizes
20 to 100 mcg

5 sizes in the shop

Contents 12 sections
  1. At a glance
  2. What is T3?
  3. How it works
  4. Dosing
  5. What the research shows
  6. Research status
  7. Side effects
  8. Storage
  9. Related compounds
  10. FAQ
  11. Glossary
  12. Sources

What is T3?

T3 (liothyronine) is the synthetic form of triiodothyronine, the active thyroid hormone. While the thyroid mainly produces the storage hormone T4 that the body must first convert to T3, liothyronine acts immediately and directly. It raises basal metabolic rate and speeds up metabolism - used in research contexts for fat burning.

This information is for research and educational purposes only. No medical advice.

Comparison

T3, T4 and other paths

Thyroid hormones and one compound with a different path to energy use, as described in the TPD lexicon. Tap a card to open its entry.

Active thyroid hormone T3 (Liothyronine) TR
Storage form, becomes T3 T4 (Levothyroxine) TR Different path to energy use Clenbuterol

T4 is the storage form and is converted to T3 in the body, T3 acts directly. About a third of the amount gives the same TSH suppression. 28

How it works

T3 binds thyroid hormone receptors in virtually every cell and raises the metabolic rate - cells burn more energy. That markedly increases calorie consumption at rest. The catch: T3 is not selective for fat. At too high a dose the body also burns muscle tissue. On top of that, the body's own thyroid production is suppressed.

Where it acts

Where T3 acts

Tap a point or a card. Colour = thyroid hormone receptor, grey = observed in studies.

  1. Pituitary

    TSH lowest after 12 hours, below baseline for 2 to 3 days3

    TR
  2. Heart

    Heart rate briefly raised 5 hours after a dose3

    TR
  3. Blood lipids

    LDL -13.3%, ApoB -18.3% compared with T41

    Observed in trials
  4. Body weight

    About 2 kg less than with T4 after 6 weeks1

    Observed in trials

Targets in the ChEMBL database8

Compound CHEMBL1201119: one recorded mechanism, agonist with direct binding to the human receptor.

Thyroid hormone receptor alpha and beta
Gene
THRA, THRB
UniProt
P10827, P10828
ChEMBL
CHEMBL2111462
Action
Agonist, direct binding
Transparent cell with warmly glowing mitochondria around the edge, two blue molecules with purple iodine atoms in the nucleus
T3 acts in the cell nucleus and drives energy metabolism. Symbolic image.

Dosing

Dosing

Dosing, titration and syringe

Loading the dosing information …

  • Getting started and stepping up, drawn as a staircase
  • Units on the U-100 syringe for every step
  • How long one vial lasts

What the research shows

The Celi study (J Clin Endocrinol Metab, 2011) compared liothyronine directly with levothyroxine (T4) and showed that T3, at an equivalent dose, led to greater weight loss and a better lipid profile, with no differences in cardiac function or insulin sensitivity. That confirms the direct metabolic effect.

Study results

What the trials measured

Interactive: switch tabs, tap points, show or hide arms. Arms are named relative to each other, no dosing advice.

Weight and blood lipids with T3 instead of T41

Difference from the T4 phase, same TSH, in percent

Body weight1 70.6 against 68.5 kg
T3 instead of T4 -3.0%
Total cholesterol1
T3 instead of T4 -10.9%
LDL cholesterol1
T3 instead of T4 -13.3%
Apolipoprotein B1
T3 instead of T4 -18.3%

14 people with hypothyroidism, 6 weeks each of T3 and T4 in random order, same target TSH. Weight calculated by us from the mean values, blood lipids as mean values in the abstract. HDL, heart rate, blood pressure, exercise capacity and insulin sensitivity did not differ.

How fast T3 comes and goes34

Hours after a single dose, healthy people

Until peak3 12 healthy people
Single dose of T3 2.5 h
Until peak, second study4 12 healthy people, calculated from 120 minutes
Single dose of T3 2.0 h
Half-life in the blood3
Single dose of T3 22.0 h

Heart rate rose briefly 5 hours after the dose. TSH reached its lowest point after 12 hours and stayed below baseline for 2 to 3 days. Depending on the chosen time points a longer half-life results, the US label gives a biological half-life of about 2.5 days.

Antidepressant with or without T35

153 adults, 8 weeks, share in percent

Response5
Antidepressant plus T3 61.8%
Antidepressant plus placebo 65.0%
Remission5
Antidepressant plus T3 40.8%
Antidepressant plus placebo 50.6%

No difference between the groups. The authors do not support routinely adding T3 at the start of treatment.

Glass tissue piece with round follicles lined by purple cells, resembling thyroid tissue
Studies on hypothyroidism, metabolism and the heart. Symbolic image.

Research status

Where the research stands

Liothyronine has been approved since 1956 and the registry lists about 900 trials with it. Here is a selection, including all trials whose results appear above. As of 26 Oct 2026. 9

  1. Preclinical Action on the thyroid hormone receptor
  2. Phase 1 no registered trial
  3. Phase 2 3 trials, 3 completed
  4. Phase 3 3 trials, 2 completed
  5. Approval in the USA since 1956
220 participants in phase 3 Planned figures for running trials Approved: in the USA liothyronine has been marketed as a prescription tablet since 1956 (Drugs@FDA). ChEMBL lists phase 4 as the highest phase.6
Show all 7 phase 2, 3 and 4 trials on a timelineHide timeline 6 completed 1 running 3 with results
Phase 2
Phase 3
Phase 4

Bar: start to primary endpoint (planned for running trials). Green = completed, blue = progress until 26 Oct 2026.

Side effects

  • Muscle loss: at too high a dose T3 also burns muscle - so in research it is often combined with anabolics to protect muscle.
  • Heart: palpitations, raised pulse, arrhythmia risk on overdose.
  • Thyroid suppression: your own production winds down - hence the slow taper.
  • Sweating, nervousness, feeling hot.

Storage

Store dry and protected from light. Out of reach of children.

Three white tablet bottles and a folded light blue towel on a white shelf
Dry and protected from light in the closed original container. Symbolic image.

Related

Related compounds

Same targets or a different path: all in the TPD lexicon.

T3 (Liothyronine)
T4 (Levothyroxine) Levothyroxine, the precursor of T3 Clenbuterol Different compound focused on energy use

FAQ

Common questions about T3 (Liothyronine)

Short answers, each backed by the sources below.

Is liothyronine approved?

Yes. In the USA it has been approved as a prescription tablet since 8 May 1956 and ChEMBL lists phase 4 as the highest phase. It is used for an underactive thyroid.678

How does T3 differ from T4?

T4 is the storage form that the body first converts to T3. T3 binds directly to the receptors. In a study after thyroid removal about a third of the amount of T3 gave the same TSH (ratio 0.36).28

What does the study on weight and blood lipids show?

14 people with hypothyroidism took T3 or T4 for 6 weeks each, adjusted to the same TSH. With T3 they weighed 68.5 instead of 70.6 kg on average, total cholesterol fell by 10.9 percent, LDL by 13.3 percent and ApoB by 18.3 percent. Heart rate, blood pressure, exercise capacity and insulin sensitivity did not differ.1

How fast does T3 act and how long does it last?

After a single dose T3 peaked in the blood after 2.5 hours and the half-life was about 22 hours. Heart rate rose briefly after about 5 hours and TSH stayed lowered for 2 to 3 days. The US label gives a biological half-life of about 2.5 days.37

What about the heart?

In the single-dose study heart rate rose temporarily above baseline a few hours after the dose. In another study researchers saw no change in heart rate, blood pressure or cardiac ultrasound during the first 4 hours after a high single dose. Over 6 weeks cardiac function did not differ between T3 and T4.341

Does T3 help with depression?

In a study of 153 adults T3 added to an antidepressant brought no benefit: response 61.8 against 65.0 percent and remission 40.8 against 50.6 percent with placebo.5

Glossary

Terms explained briefly

Dotted words in the text open these explanations directly.

ApoB
Apolipoprotein B: protein on every LDL particle, a measure of the number of these particles.
Basal metabolic rate
Energy the body uses at complete rest.
Half-life
Time until half of a substance in the blood has been broken down.
Hypothyroidism
Underactive thyroid: the thyroid makes too little hormone.
LDL
Cholesterol carrier in the blood that brings fat into the vessels. High LDL is considered a risk for the vessels.
T3 (triiodothyronine)
The active thyroid hormone with three iodine atoms. It controls the energy turnover of almost all cells.
T4 (levothyroxine)
Storage form of thyroid hormone with four iodine atoms. The body converts it into T3 as needed.
Thyroid hormone receptor
Receptor in the cell nucleus with alpha and beta forms. It switches on genes for energy metabolism.
TSH
Pituitary hormone that drives the thyroid. Plenty of thyroid hormone lowers TSH.

Evidence

Sources

Show all 10 sourcesHide sources 5 Studies1 Trial registry4 Database checked 26 Oct 2026

Studies

  1. T3 against T4 in hypothyroidism: weight and blood lipids

    Celi et al., Journal of Clinical Endocrinology and Metabolism, 2011

    DOI 10.1210/jc.2011-1329 PMID 21865366 NCT00106119
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  2. How much T3 equals T4? Study after thyroid removal

    Celi et al., Clinical Endocrinology, 2010

    DOI 10.1111/j.1365-2265.2009.03700.x PMID 20447070 NCT00106119
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  3. Single dose of T3: blood levels, heart rate and TSH over 4 days

    Jonklaas et al., Therapeutic Drug Monitoring, 2015

    DOI 10.1097/FTD.0000000000000113 PMID 24977379
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed, PMC (Volltext)
  4. Acute effects of T3 on the heart and energy use in healthy people

    Chen et al., Frontiers in Endocrinology, 2022

    DOI 10.3389/fendo.2022.843539 PMID 35295986 NCT03098433
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  5. T3 as an add-on to an antidepressant

    Garlow et al., Journal of Psychiatric Research, 2012

    DOI 10.1016/j.jpsychires.2012.08.009 PMID 22964160 NCT00208702
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed

Trial registry

  1. Trials on ClinicalTrials.gov

    ClinicalTrials.gov, U.S. National Library of Medicine

    Checked 26 Oct 2026 via ClinicalTrials.gov API v2

Database

  1. US approval (NDA 010379), prescription

    Drugs@FDA, U.S. Food and Drug Administration

    Checked 26 Oct 2026 via openFDA-API
  2. US label for liothyronine (pharmacokinetics)

    DailyMed, U.S. National Library of Medicine

    Checked 26 Oct 2026 via openFDA-API, DailyMed
  3. Mechanism and approval in ChEMBL

    ChEMBL, EMBL-EBI (CHEMBL1201119)

    Checked 26 Oct 2026 via ChEMBL
  4. Structure of liothyronine in PubChem

    PubChem, U.S. National Library of Medicine (CID 5920)

    Checked 26 Oct 2026 via PubChem

In the shop

T3 (Liothyronine) in the catalog

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