Diiodothyroacetic dcid

Other Names

2,6-Diiod-4-carboxymethyl-4'-hydroxy-diphenylether2-[4-(4-Hydroxyphenoxy)-3,5-diiodophenyl]acetic acid3,5-Diiodo Thyroacetic Acid3,5-Diiodothyroacetic acid4-(4-hydroxyphenoxy)-3,5-diiodo-benzeneacetic acid4-(4-Hydroxyphenoxy)-3,5-diiodobenzeneacetic acid4-(4-Hydroxyphenoxy)-3,5-diiodophenylacetic acidBenzeneacetic acid, 4-(4-hydroxyphenoxy)-3,5-diiodo-DIACDiiodothyroacetic acidLevothyroxine impurity 5Levothyroxine Sodium Impurity 14NSC 90463T2ATA2Thyroacetic acid[4-(4-Hydroxy-phenoxy)-3,5-dijod-phenyl]-essigsaeure[4-(p-Hydroxyphenoxy)-3,5-diiodophenyl]acetic acid

Synopsis

Diiodothyroacetic Acid (DITA / Diac / TA2): A Comprehensive Reference Article

1. Identity: Chemical and Structural Characterization

1.1 Nomenclature and Synonyms

Diiodothyroacetic acid (DITA) most commonly refers to the isomer 3,5-diiodothyroacetic acid, designated systematically as 4-(4-hydroxyphenoxy)-3,5-diiodo-benzeneacetic acid. Its formal IUPAC name is 4-(4-hydroxyphenoxy)-3,5-diiodo-benzeneacetic acid, and it carries the synonyms Diac, NSC 90463, T2A, and TA2. Its CAS Registry Number is 1155-40-4.

A second structural isomer, 3,3′-diiodothyroacetic acid (abbreviated 3,3′-TA2 or rDiac), also exists. The chemical term "iodothyroacetic acid" may refer to, but is not limited to, 3,3′-diiodothyroacetic acid and 3,5-diiodothyroacetic acid. These isomers differ in the ring positions of the two iodine atoms on the diphenyl ether (thyronine) backbone.

The molecular formula of 3,5-diiodothyroacetic acid is C₁₄H₁₀I₂O₄, with a formula weight of 496.0 g/mol.

1.2 Relationship to the Parent Thyroid Hormones

Thyroid hormones are iodothyronines that are derivatives of the amino acid tyrosine. T4 is the major thyroid hormone and has four iodine atoms, while T3 is less abundant and has three iodine atoms. 3,3′,5,5′-Tetraiodothyroacetic acid (Tetrac) and 3,3′,5-triiodothyroacetic acid (Triac) are acetic acid analogs of thyroid hormones. Diiodothyroacetic acid (Diac) occupies the next position in this series, representing an acetic acid analog bearing two iodine atoms rather than three or four.

3,5-Diiodothyroacetic acid (Diac) is the acetic acid variant of thyroxine. More precisely, it is structurally derived from the thyronine skeleton through substitution of the alanine side chain with an acetic acid moiety and retention of only the 3- and 5-position iodines of the phenyl ring.

1.3 Physical Properties and Available Forms

Diac is a crystalline solid with solubility of 30 mg/ml in DMF, 30 mg/ml in DMSO, and 0.33 mg/ml in DMSO:PBS (pH 7.2) at a 1:2 ratio. Reported purity of research-grade material is greater than 98%. It is supplied as a pure powder for research purposes. As a supplement ingredient, it has appeared in capsule and tablet formulations, though no standardized pharmacopeial monograph exists for diiodothyroacetic acid specifically.

2. Natural Source and Endogenous Occurrence

2.1 Endogenous Status in Humans

Oxidative deamination of thyroid hormones generates iodoacetic acids (TA), including tetraiodothyroacetic acid (Tetrac, TA4), Triac, and diiodothyroacetic acid (Diac, TA2), which are habitually present at low concentrations in human serum. Diac is therefore not an exogenous plant-derived botanical or mineral but rather an endogenous thyroid hormone metabolite belonging to the class of thyroacetic acids.

Diiodothyroacetic acid is a direct naturally occurring metabolite of T3, Triac, and T2. The sulfate conjugate 3,3′-diiodothyroacetic acid (3,3′-TA2S) was discovered in plasma, and occasionally in bile, of 6-propyl-2-thiouracil-treated rats after administration of T3; the significant plasma 3,3′-TA2S levels illustrate the physiological relevance of this T3 metabolite.

The possibility of diiodothyroacetic acid (T2AC) can be inferred from its structure and metabolic characteristics, although only T4AC and T3AC have been detected in human plasma at 40–700 pmol/L so far. A 2024 study using isotope-dilution LC-MS/MS reported for the first time direct quantification of T2A (diiodothyroacetic acid) levels in human serum. To the knowledge of the study's authors, it was the first time data for T2A as well as for free thyroid hormone metabolite levels in human serum were published in the literature.

2.2 Biological Distribution

Thyroxines and iodotyrosines are found not only in mammals, but also in sponges, ascidians, gorgonians, marine algae, and insects. The broader class of iodinated thyronine-related compounds is thus widely distributed in nature. Iodothyroacetic acids, including diiodothyroacetic acid (Diac), are increasingly being recognized as biologically active compounds among thyroid hormone-related metabolites.

3. Biosynthetic Pathway and Formation

3.1 Mechanism of Natural Biosynthesis

The alternate pathways of thyroid hormone metabolism — including the oxidative deamination and decarboxylation of the alanine side chain to form iodothyroacetic acids — provide additional mechanisms for regulating the supply of active hormone. The major thyroid hormone secreted by the thyroid gland is thyroxine (T4), and triiodothyronine (T3), formed chiefly by deiodination of T4, is the active hormone at the nuclear receptor.

Thyroid hormone metabolites (THM) include thyronamines (TAMs), resulting from TH decarboxylation, and thyroacetic acids (TAc) resulting from the deamination of TAMs. The more highly iodinated thyroacetic acids Triac and Tetrac, which are biologically active oxidative deamination metabolites of T3 and T4 respectively, have been known for several decades, though the biosynthetic origins of these compounds and the enzymes involved in the conversion are not completely clear.

The biosynthetic pathway of diiodothyroacetic acid is unique in that it possesses several direct pathways to different thyroid hormones in contrast to other acetic acid analogs such as T3A (Triac) and T4A (Tetrac). Diiodothyroacetic acid has direct reversible pathways to T3, Triac, and T2.

Early studies also found sulfated 3,3′-diiodothyroacetic acid (3,3′-T2ACS) in human and rat serum after T3AC (Triac) administration, suggesting that further metabolism of Triac involves deiodination and sulfation. Sulfation of Triac and 3,3′-T2AC has been observed in rat liver microsomes, and sulfation of Triac is believed to promote its deiodination, making Triac the likely precursor of most thyroacetic acids and their conjugates.

Iodothyroacetic acids are thyroid metabolites which come from the corresponding iodothyronines, which undergo, at tissue and principally at liver and kidney level, a degradation of the alanine chain by oxidation, probably after passage in the form of iodothyropyruvic acids.

4. Historical and Research Context

4.1 Early Scientific Investigation of Iodothyroacetic Acids (1950s–1960s)

There is no documented history of human traditional or ethnobotanical use of diiodothyroacetic acid as a discrete preparation. As a natural endogenous metabolite rather than a plant-derived botanical, its history is entirely scientific. The compound entered the scientific literature in the context of synthetic thyroid hormone analog research.

Beginning in the 1950s, extensive efforts were made to develop thyroid hormone analogs that could utilize the cholesterol-lowering property of thyroid hormones in euthyroid individuals without affecting the heart. Thyroid hormone had been established to have the unique property of lowering cholesterol in hypothyroid individuals and improving cardiac performance.

Only the 3,5,3′,5′-tetraiodothyroacetic acid (TETRAC) and the 3,5,3′-triiodothyroacetic acid (TRIAC) were demonstrated with certainty in the organism in earlier studies dating to the 1950s. Diiodothyroacetic acid was considered alongside these congeners but was less thoroughly characterized at that time.

As early as the mid-20th century, 3,5-diiodothyroacetic acid (Diac) was included in comparative studies of cholesterol metabolism alongside thyroid hormones and multiple thyroxine analogues including D-thyroxine, Triac, Tetrac, Triprop, and Tetraprop.

A 1965 study published in the Henry Ford Hospital Medical Journal by Beher et al. examined the effects of 3,5-diiodothyroacetic acid and 3,3′,5-triiodothyroacetic acids on the time course of steroid C-14 metabolism in the rat. This represents one of the earliest specific published investigations of Diac's biological activity.

Although the presence of 3,3′-diiodothyroacetic acid (rDiac) in the organism had not been demonstrated as of early patent literature, it could be thought that these compounds were hormonal metabolites since the corresponding iodothyronines rT3 and rT2 are compounds normally present in the organism. Studies on the biological activity of Tetrac and Triac showed that their properties were only slightly different from those of the iodothyronines from which they derive, though the intensity of responses indicated that their action was inferior to that of their precursors.

4.2 Dietary Supplement Context

Diiodothyroacetic acid is a direct naturally occurring metabolite of T3, Triac, and T2, which has never been investigated or sold as a new drug; therefore, it may be sold as a dietary supplement. This claim, contained in a U.S. patent, reflects the regulatory rationale under which DITA has appeared commercially. No regulatory approval as a pharmaceutical drug has been granted for diiodothyroacetic acid in any major jurisdiction, and no formal government health agency (NIH ODS, EMA, EFSA) has issued a specific monograph or evaluation for this substance.

5. Key Constituents and Active Compounds

Diiodothyroacetic acid is itself the active entity; it is a single, chemically defined small molecule. Its structure determines its mechanism of action. The key features of its structure relevant to activity are:

A diphenyl ether (thyronine) backbone consisting of two phenyl rings connected by an oxygen bridge, derived from the thyroxine skeleton.
Two iodine atoms at the 3- and 5-positions (for the Diac isomer) of the inner phenyl ring.
A 4′-hydroxyl group on the outer phenyl ring, which is essential for antioxidant activity against LDL oxidation.
An acetic acid side chain in place of the alanine side chain present on T3 and T4.

The antioxidant potency of thyroid compounds (including Diac) is supported by their 4′-hydroxy-diphenylether structure, and it varies according to the nature and the position of substituents in this structure.

6. Mechanisms of Action

6.1 Thyroid Hormone Receptor (TR) Interactions

Thyroid hormone (T3) regulates gene expression by binding to high-affinity nuclear receptors. Thyroid hormone receptors (TRs) recognize specific response element sequences in the promoters of T3-target genes and activate or repress transcription in response to hormone. Diac, as a structurally similar analog, interacts with the same receptor system, though typically with lower affinity than T3.

Efforts to develop thyroid hormone analogs culminated in the identification of compounds that selectively bind to β1-type nuclear thyroid hormone receptors (TRs), which are responsible for cholesterol-lowering activity, without activating α1-type receptors in the heart. Diac is classified within the broader family of thyromimetic compounds and thyroid hormone analogs, all of which share this general receptor-based framework.

There is both experimental and clinical evidence indicating that thyroid analogs act differently than thyroid hormones; the details of their mechanism of action have not been completely elucidated. A number of potential mechanisms have been reviewed, including serum protein binding, tissue disposition, receptor binding, and gene activation.

6.2 Precursor Hormone Activity

Diiodothyroacetic acid acts as a precursor hormone resulting in specific small increases in T3, Triac, and T2. Small increases in T3 facilitate protein synthesis for muscle anabolism. The reversible nature of its metabolic connections to multiple thyroid hormones is proposed as a mechanistic basis for its alleged body-composition effects, though these claims derive entirely from patent literature and lack independent clinical verification.

Diiodothyroacetic acid exerts a direct enhancement of metabolic rate via an increase in oxygen consumption and body temperature. This increase in metabolic rate results in an enhancement of the utilization of orally consumed nutrients. Again, these mechanistic assertions are from a U.S. patent filing and have not been independently replicated in peer-reviewed clinical trials specifically for Diac.

6.3 Lipid Metabolism Mechanisms

The ability of thyroid hormone to lower cholesterol when given to hypothyroid individuals prompted efforts to design analogs that take advantage of these properties. This action is the result of an accelerated LDL-cholesterol clearance rate. Triiodothyronine increases levels of both the hepatic LDL receptor and its mRNA, and additional thyroid hormone actions on lipid metabolism include increasing the activity of lipoprotein lipase. Diac is proposed to exert analogous, though weaker, effects.

Hypercholesterolemia in hypothyroidism is mainly due to a reduction in low-density lipoprotein (LDL) receptor activity, accompanied by concomitant diminishing control by T3 of sterol regulatory element-binding protein 2 (SREBP-2), which modulates cholesterol biosynthesis by regulating rate-limiting enzyme HMG-CoA reductase activity.

6.4 Antioxidant Mechanisms

Thyroid hormones and structural analogues have been shown to protect LDL against oxidation induced by Cu²⁺, endothelial cells, or macrophages in vitro. The antioxidant potency of thyroid compounds is supported by their 4′-hydroxy-diphenylether structure, and these compounds also have free radical scavenging capacity. Free radical scavenging is not the only mechanism responsible for the antioxidant effect of thyroid compounds on copper-induced LDL oxidation; for example, Triac is more antioxidant than T3 on LDL oxidation induced by copper ions, suggesting these compounds act by different mechanisms.

7. Scientific Evidence by Area of Use

7.1 Cholesterol and Lipid Metabolism

Preclinical (animal/in vitro) evidence:

3,5-Diiodothyroacetic acid inhibits copper-induced lipid peroxidation of LDL isolated from human plasma when used at a concentration of 1 µM. At a dose of 5 mg/100 g diet, it increases urinary and fecal excretion of sterols, as well as prevents high-fat diet-induced increases in liver lipid and cholesterol levels in rats.

Diac treatment prevents elevated liver cholesterol in rats fed a high-fat diet and reduces previously elevated liver cholesterol to control levels. These are preclinical rodent findings and do not constitute evidence of efficacy in humans.

Human/clinical evidence:

Formulations containing dosages in the range of 100 mg/day have been reported to decrease serum cholesterol and produce calorigenic effects in humans. This claim appears in research chemical supplier documentation and appears to reference older mid-20th century literature. No modern randomized controlled trial specifically testing isolated 3,5-diiodothyroacetic acid in human subjects for cholesterol reduction has been identified in peer-reviewed databases (PubMed/PMC). The evidence for cholesterol-lowering in humans must therefore be characterized as very preliminary, based on older and not independently replicated data.

7.2 LDL Oxidation Protection (Antioxidant Activity)

In vitro evidence:

Formulations containing Diac protect human LDL cholesterol from oxidation in vitro. 3,5-Diiodothyroacetic acid inhibits copper-induced lipid peroxidation of LDL isolated from human plasma when used at a concentration of 1 µM. This antioxidant activity has been characterized in cell-free and cell culture systems. No human clinical trial data are available specifically for Diac's LDL antioxidant effects in vivo.

7.3 Body Composition and Metabolic Rate

Proposed mechanism (patent-level claims, not clinical trial data):

The proposed mechanism for effects on body composition is that effective administration of diiodothyroacetic acid shifts the proportion between lean body mass and adipose tissue in favor of lean body mass due to its location in the thyroid biosynthetic pathway. Its proposed ability to shift the proportion between lean body mass and adipose tissue is attributed to small increases in T3 for enhanced protein synthesis and muscle tissue accretion.

Evidence strength: These claims derive from a U.S. patent (US 7,919,533) and have not been tested in published, peer-reviewed, randomized controlled trials in humans. They must be characterized as speculative/unverified at the clinical level.

Context from related compound research: Studies on the closely related compound 3,5-diiodothyronine (T2) — the precursor of Diac — are informative. In general, 3,5-diiodothyronine (3,5-T2) increases the resting metabolic rate and oxygen consumption, exerting short-term beneficial metabolic effects on rats subjected to a high-fat diet. However, T2 administration also leads to suppression of the HPT axis, increased food intake, and cardiac hypertrophy. A particular point of concern is the observation that the lower dose of T2 exerts negligible effects on adiposity and metabolic outcomes, yet results in a marked suppression of the HPT axis leading to reduced levels of circulating T4 and T3, with unknown long-term consequences. The implication is that, for a given dose, the detrimental effects of T2 on the HPT axis may preferentially occur before the intended metabolic ones. These findings for T2, while not directly testing Diac, are relevant given that Diac is proposed to act in part through conversion to T2 and T3.

7.4 Thyroid Axis Modulation and TSH

The administration of T3AC (Triac) in humans can suppress TSH levels and thyroidal secretion, mirroring the regulatory effect of T3 on the hypothalamus-pituitary-thyroid axis. Given that Diac is proposed to serve as a precursor to Triac, it would be reasonable to hypothesize that Diac could exert TSH-suppressive effects, but no direct clinical evidence for Diac-specific TSH suppression in humans has been published in the peer-reviewed literature identified by this review.

It has been suggested that intracellular rerouting of T3 to Triac (TA3) during fasting is responsible for the maintenance of normal serum TSH levels in the presence of low T3 concentrations. This dynamic nature of the interconversion pathway means that any administered Diac could influence the HPT axis through its downstream metabolites.

7.5 Steroid Metabolism

The 1965 Beher et al. study investigated the effects of 3,5-diiodothyroacetic and 3,3′,5-triiodothyroacetic acids on steroid C-14 metabolism in the rat. These effects were examined as part of a broader inquiry into steroid metabolism and thyroid hormone analog pharmacology. The evidence from this early work is not replicated in modern peer-reviewed clinical trials and must be regarded as preliminary and animal-based only.

8. Body Systems and Health Areas

Based on available evidence, diiodothyroacetic acid is associated with the following body systems:

Endocrine/Thyroid System: Diiodothyroacetic acid has direct reversible pathways to T3, Triac, and T2, placing it within the network of the hypothalamus-pituitary-thyroid (HPT) axis. Its administration is expected to have consequences for endogenous thyroid hormone levels and TSH regulation.
Cardiovascular System / Lipid Metabolism: 3,5-Diiodothyroacetic acid lowers blood cholesterol concentrations in preclinical models. Its inhibition of copper-induced LDL oxidation is relevant to cardiovascular risk biology.
Hepatic (Liver) System: Iodothyroacetic acids undergo degradation principally at the liver and kidney level. The liver is the primary site of TH analog action on cholesterol metabolism via hepatic LDL receptor regulation.
Metabolic System (Energy/Thermogenesis): Diiodothyroacetic acid exerts a direct enhancement of metabolic rate via an increase in oxygen consumption and body temperature (claimed in patent literature; clinical evidence absent).
Musculoskeletal System (proposed): Small increases in T3 derived from Diac precursor activity are claimed to facilitate protein synthesis for muscle anabolism.

9. Dosage Forms and Reported Dosages

No standardized pharmaceutical dosage form for diiodothyroacetic acid exists, as it is not approved as a drug. The following dosage information has been identified in primary sources:

Dietary supplement use (patent-specified): In the method of promoting lean body mass described in U.S. Patent 7,919,533, diiodothyroacetic acid should be administered in a daily dose of from about 1 mcg to about 6 mg. It is preferred that the daily dose be divided into a plurality of individual doses, with three to six individual doses preferred.
Human calorigenic and cholesterol-lowering effects (historical reference): Formulations containing dosages in the range of 100 mg/day have been reported to decrease serum cholesterol and produce calorigenic effects in humans. This figure appears in research chemical databases referencing older literature. The source study has not been independently verified in this review through peer-reviewed search results.
Animal (rat) dietary study: In rats, 3,5-diiodothyroacetic acid at 5 mg/100 g diet increased urinary and fecal excretion of sterols and prevented high-fat diet-induced increases in liver lipid and cholesterol levels.
In vitro LDL oxidation inhibition: Inhibition of copper-induced lipid peroxidation of LDL was demonstrated at a concentration of 1 µM.

It must be emphasized that no randomized controlled human clinical trial establishing a safe and effective dosage regimen for diiodothyroacetic acid as a dietary supplement has been identified in this review.

10. Safety Considerations and Known Interactions

10.1 HPT Axis Suppression Risk

The most scientifically grounded safety concern with diiodothyroacetic acid — and closely related thyroid hormone metabolites — is suppression of the hypothalamus-pituitary-thyroid (HPT) axis. The administration of T3AC (Triac) in humans can suppress TSH levels and thyroidal secretion. Since Diac is metabolically connected to Triac via direct reversible pathways, Diac supplementation may exert similar TSH-suppressive effects. Studies on the precursor compound T2 provide direct animal evidence: administration of 3,5-T2 suppressed thyroid function, reducing not only thyroid iodide uptake but also thyroperoxidase, NOX4, and DIO1 activities, while serum TSH, T3, and T4 were reduced in a T2 dose-dependent manner.

10.2 Thyromimetic Side Effects

The therapeutic use of thyroid hormones and analogs for treatment of hyperlipidemia is limited due to deleterious side effects from TR activation in extrahepatic tissues, leading to altered cardiovascular function, muscle wasting, and bone loss. Diac, as a thyromimetic analog, would be expected to carry analogous risks, particularly at higher doses. Pharmacologic use of natural thyroid hormones for cholesterol lowering is limited by their actions on other organs, including the heart, bone, and brain, where there can be side effects of excessive thyroid hormone action.

10.3 Lipokinetix Hepatotoxicity Case Series

A significant safety signal relevant to thyroid hormone metabolites in supplements comes from the Lipokinetix case series. A dietary supplement used for weight loss called Lipokinetix was composed of norephedrine, sodium usniate (usnic acid), diiodothyronine, yohimbine, and caffeine; it was associated with acute hepatitis, including fulminant hepatic failure requiring liver transplantation. Importantly, this product contained diiodothyronine (T2), not diiodothyroacetic acid (Diac/T2A), and the hepatotoxicity was attributed primarily to usnic acid within the combination. However, this case series illustrates the general risk profile when iodothyronine-class compounds are included in multi-ingredient weight-loss supplements.

10.4 Cardiovascular Considerations

Triac (a direct metabolic neighbor of Diac) has an even higher affinity for thyroid hormone receptors than T3. When administered at concentrations comparable to those of T4 and T3, these compounds show T3-like effects. At doses producing thyromimetic effects, thyroid hormone analogs as a class have been associated with tachycardia, cardiac hypertrophy, arrhythmias, and muscle wasting in animal studies. At high doses in animal studies, thyroid hormone analogs have caused cardiac hypertrophy and resulted in elevated metabolic rate and body temperature.

10.5 Absence of Comprehensive Human Toxicology Data

No formal toxicology dossier, long-term human safety study, or official risk assessment specific to 3,5-diiodothyroacetic acid as a dietary supplement has been published by any regulatory authority (FDA, EMA, EFSA, Health Canada). The compound's endogenous nature means it circulates at very low concentrations naturally, but the pharmacological consequences of oral supplementation at doses many orders of magnitude higher than endogenous levels are not well characterized in peer-reviewed literature.

10.6 Potential Drug Interactions

Given its thyromimetic mechanism of action, diiodothyroacetic acid would be expected to interact pharmacologically with:

Thyroid hormone replacement therapy (levothyroxine, liothyronine): potential additive thyromimetic effects and TSH suppression.
Anticoagulants (warfarin): Thyroid hormones and analogs potentiate the effect of anticoagulants by mechanisms involving accelerated metabolic clearance of clotting factors; this is a recognized class effect for thyromimetics.
Antidiabetic drugs: Anti-insulin effects on glucose metabolism are elicited by some thyroid hormone analogs at slightly higher doses, suggesting a potential for interference with blood glucose control.

None of these interactions have been specifically studied for diiodothyroacetic acid in controlled human studies identified by this review.

11. Overall Evidence Assessment

Diiodothyroacetic acid is a structurally and metabolically well-defined endogenous thyroid hormone metabolite whose scientific characterization is still incomplete. Its natural occurrence at low concentrations in mammals is established by its detection in biological fluids and its place within the mapped thyroid hormone metabolic network. Its biochemistry — as an acetic acid analog of T2 with reversible metabolic connections to T3, T2, and Triac — provides a plausible mechanistic rationale for proposed effects on lipid metabolism and metabolic rate.

Preclinical evidence in rodents for cholesterol-lowering and LDL oxidation protection exists and is consistent with broader thyroid hormone analog pharmacology. However, no randomized, placebo-controlled human clinical trials specifically testing 3,5-diiodothyroacetic acid as an isolated agent have been identified in PubMed/PMC. Human efficacy and safety data are largely absent or limited to very old, small, and not independently replicated studies. Claims for body-composition benefits derive entirely from patent literature and lack peer-reviewed support. The compound's thyromimetic nature means safety risks — including HPT axis suppression, cardiac effects, and bone effects — are scientifically plausible and not adequately characterized for supplemental use.

References

Health Conditions

Health conditions that Diiodothyroacetic dcid may help support.

No conditions available.

Body Systems

Body systems that Diiodothyroacetic dcid may help support.

No body systems available.