IngredientsThe term iodotyrosine refers to a family of iodinated derivatives of the amino acid L-tyrosine. Two principal members of this family are of biochemical and nutritional significance:
Iodotyrosine is a product from the iodination of tyrosine. In the biosynthesis of thyroid hormones (thyroxine and triiodothyronine), tyrosine is first iodized to monoiodotyrosine. Both MIT and DIT are therefore organic iodine compounds in which one or two iodine atoms, respectively, are covalently bonded to the aromatic ring of the amino acid tyrosine.
MIT and DIT occur endogenously in the thyroid glands of all vertebrates as obligatory intermediates in thyroid hormone biosynthesis. They also occur in a variety of marine and plant organisms. Plants, insects, zooplankton, and algae store iodine as mono-iodotyrosine (MIT), di-iodotyrosine (DIT), iodocarbons, or iodoproteins.
Edible seaweeds are among the most analytically well-characterized dietary sources. Within seaweed, iodine exists in multiple forms, which can generally be classified into inorganic iodine (e.g., iodide and iodate) and organic iodine (e.g., monoiodotyrosine (MIT), diiodotyrosine (DIT)). Research using reversed-phase HPLC coupled to inductively coupled plasma mass spectrometry (ICP-MS) has confirmed the presence of these compounds in specific species. Whereas iodide is the most predominant species present in Kombu (Laminaria japonica), a more complicated distribution of iodine is present in Wakame (Undaria pinnatifida). This study shows that incorporation of iodine in different seaweeds follows different metabolic pathways, notwithstanding that both belong to the same class, Phaeophyceae. The presence of iodide was proved in Kombu, while in the case of Wakame, monoiodotyrosine and diiodotyrosine are also present and probably bound to the proteins.
In all seaweeds examined, iodide was the predominant species, and minor contents of MIT (monoiodotyrosine) and DIT (diiodotyrosine) were found in most seaweeds. The iodinated amino acids in seaweeds are primarily protein-bound, and analytical recovery requires enzymatic hydrolysis. The combination of reverse-phase HPLC with ICP-MS has been used for the determination of MIT and DIT in edible seaweed; a sample pre-treatment based on ultrasound-assisted enzymatic hydrolysis was optimized for the extraction of these iodinated amino acids, and pancreatin was selected as the most adequate type of enzyme.
Iodotyrosines are found across a broad taxonomic range, including iodotyrosines in algae, sponges, corals, starfish, mollusks, annelids, crustaceans, and insects. Early research proposed that iodine could be metabolized by algae to iodinated amino acids including MIT and DIT. Early studies put forward a hypothesis that iodine could be metabolized by algae to iodinated amino acids (monoiodotyrosine β MIT, diiodotyrosine β DIT, diiodothyronine β T2 and T4); the presence of iodinated amino acids was confirmed in an enzymatic extract of Wakame algae by reversed-phase HPLC-ICP-MS.
As a dietary supplement or research material, iodotyrosines may be encountered in the following forms:
The substance "iodotyrosine" as an isolated compound was not known to pre-modern cultures. However, the iodine-rich botanical and marine materials from which it is now understood to derive were used empirically for millennia, primarily for the treatment of goiter (thyroid enlargement).
The history of thyroid disease treatment dates from around 2697 BCE when the "Yellow Emperor" Hung Ti described the use of seaweed to treat goiter. The English name "thyroid" was coined by Thomas Wharton in 1656 from the Greek word for a shield. In the 5th century AD the Chinese alchemist Ko-Hung recommended an alcoholic extract of seaweed for goiter. Treatment of goiters with marine products, such as sponges and seaweed, was common during the 14th century.
Numerous remedies for goiter were proposed in medieval Europe; some were complex mixtures of seaweed and marine sponge. The early observations, including those pertaining to treatment of goitre with preparations derived from iodine-containing sponge or seaweed, were not based on an understanding of the physiological role of the thyroid gland. These preparations were therefore empirical, and their therapeutic action is now retrospectively attributed to their iodine content, including organically bound forms such as MIT and DIT.
Bernard Courtois discovered iodine in 1811 when he noted a residual purplish ash while burning seaweed. This discovery provided the chemical basis for explaining the traditional therapeutic utility of marine preparations. Coindet, an Edinburgh-trained physician working among goitrous people in Geneva, Switzerland, considered iodine to be the active ingredient of the empiric therapies for goiter. In 1820, he gave iodine, mostly as the potassium salt, to patients with goiter, after which their goiters shrank remarkably.
Dr. David Marine's landmark experiment in 1916β1920, where iodide administration to 5,000 schoolgirls in Akron, Ohio, prevented development of endemic goiter, led to iodized salt becoming commercially available in Michigan in 1924, followed by the remainder of the USA. The specific contribution of organic iodine forms such as MIT and DIT from dietary sources was not distinguished from inorganic iodine in these early interventions.
The biochemical identification and characterization of MIT and DIT as distinct intermediates in thyroid hormone synthesis occurred across the early-to-mid 20th century, with the elucidation of the full thyroid hormone biosynthetic pathway. Failure of the iodotyrosine deiodinase enzyme leads to hypothyroidism, goiter, and mental retardation, a clinical phenotype described as early as the 1950s, whose diagnostic hallmark is the elevation of iodotyrosines in serum and urine. At that stage, MIT and DIT were recognized not merely as dietary components but as essential biochemical actors in human physiology.
Iodide is a fundamental constituent of the thyroid hormones thyroxine (T4) and triiodothyronine (T3), which are synthesized in the follicular epithelial cells from tyrosine residues present in the thyroid protein thyroglobulin. Thyroglobulin (Tg) is the master scaffold upon which MIT and DIT are formed and from which the active thyroid hormones are subsequently cleaved. Thyrocytes synthesize thyroglobulin in vast quantity, iodinating both hormonogenic and non-hormonogenic tyrosines on the protein in the follicular lumen, where it serves as the body's supply of stored iodide (mono- and di-iodotyrosine) as well as thyroid hormone (primarily T4, thyroxine).
Synthesis of thyroid hormone begins when iodide molecules enter the follicular cell of the thyroid gland via the sodiumβiodide transporter and are then transported into the colloid. Once in the thyroid gland, iodide is oxidized and is quickly associated with thyroglobulin. Organification of the iodide results in the inactive iodotyrosines, monoiodotyrosine and diiodotyrosine. The enzyme principally responsible for this organification is thyroid peroxidase (TPO). TPO utilizes HβOβ generated by the DUOX system and iodide transported to the follicular lumen (via NIS plus apical iodide transporters) to generate oxidized iodide, which becomes available to interact with protein tyrosine residues to produce 3-iodotyrosine (MIT) and 3,5β²-diiodotyrosine (DIT).
The number of iodinated tyrosine residues depends closely on the iodine supply and on thyrotropin stimulation. About 70% of thyroglobulin's iodine is in MIT and DIT.
Following organification, these tyrosine residues are coupled to form the two active thyroid hormones: thyroxine (T4) and triiodothyronine (T3). Organification and coupling occur in the matrix of thyroglobulin. The specific coupling reactions are:
Free DIT (not bound to thyroglobulin) has a feedback-modulatory role on TPO itself. DIT is a regulatory ligand for thyroid peroxidase. Inhibition of thyroglobulin iodination catalyzed by thyroid peroxidase was observed when free diiodotyrosine concentration was higher than 5 ΞΌM. This inhibition was competitive, suggesting that free diiodotyrosine interacts with the substrate site(s) of thyroid peroxidase. This competitive inhibition mechanism has been characterized in biochemical studies and indicates that MIT and DIT are not merely passive intermediates but also exercise local regulatory control over the iodination cascade.
The flavoprotein iodotyrosine deiodinase (IYD) salvages iodide from mono- and diiodotyrosine formed during the biosynthesis of the thyroid hormone thyroxine. This enzyme is structurally and catalytically unique. Iodotyrosine deiodinase (IYD) is unusual for its reliance on flavin to promote reductive dehalogenation under aerobic conditions. As implied by the name, this enzyme was first discovered to catalyze iodide elimination from iodotyrosine for recycling iodide during synthesis of tetra- and triiodothyronine collectively known as thyroid hormone.
The iodotyrosine deiodinase responsible for this activity is an NADPH-dependent flavoprotein (it is not a selenium-containing enzyme, and it should not be confused with iodothyronine deiodinase). The gene encoding it is designated IYD (also known as DEHAL1). The IYD gene (also referred to as DEHAL1) encodes iodotyrosine deiodinase, the enzyme responsible for iodine recycling in the follicular cell, to avoid leaking of iodine and loss of the precious mineral in urine.
DEHAL1 mRNA is highly expressed in the thyroid, is up-regulated by cAMP, and encodes a transmembrane protein that efficiently catalyzes the NADPH-dependent deiodination of mono- (L-MIT) and diiodotyrosine (L-DIT), with greater activity versus L-MIT. Immunohistochemistry studies showed that DEHAL1 proteins accumulate at the apical pole of thyrocytes. The deiodination reaction occurs at the apical pole of the thyrocyte and is involved in a rapid iodide recycling process at and/or close to the organification site.
Although small amounts of MIT and DIT are secreted into the circulation, most is deiodinated within the thyroid and its iodide is returned to the general pool for recycling. This mechanism is important for iodine conservation, as shown by the functional iodine deficiency of patients who have the rare defect that blocks this step.
DEHAL1 is a key factor for iodine metabolism in the thyroid gland. In periods of iodine deficiency, it allows the reuse of iodine from iodotyrosines for hormone synthesis; in iodine sufficiency, it contributes to optimal iodine storage in the thyroid gland.
The use of iodotyrosines to control development and other biological processes predates the evolution of the thyroid gland. The capacity to form and metabolize iodotyrosines is broadly distributed across the tree of life. The role of both iodide and iodinated tyrosine derivatives is currently unknown in lower organisms, yet the presence of a key enzyme in iodide conservation, iodotyrosine deiodinase (IYD), is suggested by genomic data from a wide range of multicellular organisms as well as some bacteria. A representative set of these genes has now been expressed, and the resulting enzymes all catalyze reductive deiodination of diiodotyrosine with kcat/Km values within a single order of magnitude. This implies a physiological presence of iodotyrosines (or related halotyrosines) and a physiological role for their turnover.
Iodotyrosines are highly reactive with other molecules, which may have made them important cell signaling molecules early in evolutionary history. They form spontaneously without need for enzymatic catalysts, which may have contributed to their early adoption by organisms, although enzymes make the yields significantly higher.
DIT was most effective at initiating strobilation (metamorphosis from polyp to medusa) in the cnidarian jellyfish Aurelia aurita, and this is consistent with a proposal that iodotyrosines could be evolutionary precursors of thyroid hormones. This presents a possibility that iodotyrosine deiodinase may assume a regulatory role by its ability to degrade this primitive iodinated signal.
Before evolving the ability to synthesize thyroid hormone, some organisms are thought to have obtained thyroid hormones from food sources and used the ingested thyroid hormones to regulate various physiological functions. This hypothesis is supported by evidence that many common marine food sources such as algae, sponges, and corals contain very large amounts of iodinated organic compounds.
The primary body system associated with iodotyrosines is the thyroid gland and the hypothalamic-pituitary-thyroid (HPT) axis. MIT and DIT are the obligatory biosynthetic precursors to T3 and T4, and as such they occupy a pivotal position in endocrine physiology. Tyrosine residues are iodinated to form mono- and di-iodotyrosines, which then undergo oxidative coupling to form the active hormones, which are then stored as part of the thyroglobulin molecule within the thyroid follicle. Thyroid hormone synthesis and secretion are controlled by the anterior pituitary, which secretes thyrotropin under the influence of the hypothalamic hormone thyrotropin-releasing hormone.
Because MIT and DIT are essential precursors to thyroid hormones, and because thyroid hormones are critical for normal brain maturation, defects in iodotyrosine metabolism have direct neurological consequences. Patients with DEHAL1 mutations had severe goitrous hypothyroidism, which was evident in infancy and childhood. Two patients had cognitive deficits due to late diagnosis and treatment. Exposure to TPO inhibitors during early gestation could potentially affect the development, and even moderate reduction in thyroid hormone levels during early stages can cause irreversible neurological damage leading to cognitive and learning deficits.
Since thyroid hormones regulate basal metabolic rate, thermogenesis, and somatic growth, any disruption in MIT/DIT availability that impairs T3/T4 synthesis will alter these systemic parameters. Thyroid hormones became active agents in the metamorphosis and thermogenesis of vertebrates, facilitating their adaptation to the terrestrial environment.
MIT has an additional pharmacological activity beyond the thyroid axis. 3-iodotyrosine (3-IY) inhibits tyrosine hydroxylase, which catalyzes levodopa (L-DOPA) formation from tyrosine. Tyrosine hydroxylase is the rate-limiting enzyme in catecholamine biosynthesis; MIT therefore has the potential to influence dopamine, norepinephrine, and epinephrine pathways, at least in experimental systems. This has been described primarily in in vitro and invertebrate model contexts.
Evidence type: Human clinical genetics studies. The most robust human clinical evidence involving iodotyrosines relates to loss-of-function mutations in the DEHAL1/IYD gene. DEHAL1 has been identified as the gene encoding iodotyrosine deiodinase in the thyroid, where it controls the reuse of iodide for thyroid hormone synthesis. Researchers screened patients with hypothyroidism who had features suggestive of an iodotyrosine deiodinase defect for mutations in DEHAL1. Two missense mutations and a deletion of three base pairs were identified in four patients from three unrelated families; all the patients had a dramatic reduction of in vitro activity of iodotyrosine deiodinase.
Mutations in DEHAL1 led to a deficiency in iodotyrosine deiodinase in these patients. Because infants with DEHAL1 defects may have normal thyroid function at birth, they may be missed by neonatal screening programs for congenital hypothyroidism.
The pathophysiology is well understood: Failure to deiodinate thyroid monoiodotyrosine (MIT) and diiodotyrosine (DIT) as they are released from thyroglobulin leads to severe iodine wastage, because the non-deiodinated MIT and DIT leak out of the thyroid and are excreted in urine. The patients originally described were hypothyroid, with goiters presenting at birth or shortly thereafter. Elevated iodotyrosines in plasma and urine, together with their deaminated derivatives, are the diagnostic hallmark. Hypothyroidism is presumed to result from the loss of large quantities of MIT and DIT in the urine and secondary iodine deficiency. The goiter and hypothyroidism are relieved by administration of high doses of iodine.
Evidence strength: Strong for the pathophysiological role of iodotyrosines in this rare disorder. Evidence is derived from small case series (a handful of affected families reported globally) and biochemical/genetic analyses rather than randomized trials.
Evidence type: Animal (knockout mouse) studies; emerging translational research. Recent preclinical work has investigated whether serum or urinary MIT/DIT levels could serve as early biomarkers of iodine-deficient hypothyroidism, even prior to overt clinical disease. DEHAL1 removes iodine from mono- and diiodotyrosines (MIT, DIT), making the former available for recycling and reuse in the thyroid gland to sustain thyroxine (T4) and triiodothyronine (T3) synthesis. Loss-of-function DEHAL1 mutations were shown to cause severe goitrous hypothyroidism and mental retardation.
The model of dehalogenase deficiency presented supports the proposed paradigm that genetic factors mediate the relation between iodine nutrition and iodine status (reserves). From a translational perspective, elevated iodotyrosines are biomarkers for the risk and vulnerability to iodine-deficient hypothyroidism in asymptomatic individuals with defective iodine recycling.
While urinary loss of iodotyrosines could represent early biomarkers for the disorder, its determination in fluids remains a technical challenge. Since iodinated tyrosine can be excreted in the urine, a recent study proposed detection of MIT and DIT in urine as an alternative screening method. However, the presented concentrations are on a scale of 10β»Β² ng per g of urine, and DIT is nearly undetectable in most samples. Furthermore, recent studies report that excreted MIT/DIT can only account for a small fraction of the total amount of iodinated compounds lost in the urine.
Evidence strength: Preliminary and predominantly preclinical (mouse models). Human studies are limited by the technical difficulty of measuring MIT and DIT at physiologically relevant concentrations. No clinical trials have validated iodotyrosines as routine diagnostic biomarkers.
Evidence type: Biochemical/mechanistic only. No human clinical trials. Some contemporary dietary supplement products contain DIT, with the stated rationale being provision of a thyroid hormone biosynthetic precursor, particularly in populations at risk for iodine deficiency. The biochemical logic is that DIT can supply both an iodine source and a tyrosine scaffold. While DIT itself has not been as extensively studied in clinical trials as the end-product thyroid hormones, its contribution to thyroid health is supported by decades of biochemical research. DIT is sometimes included in nutritional supplements aimed at supporting thyroid function, based on the rationale that providing precursors may aid in hormone synthesis, especially in populations at risk for iodine deficiency.
Evidence strength: No controlled human clinical trials have been identified in the peer-reviewed literature that specifically evaluate the efficacy of exogenous MIT or DIT supplementation on thyroid hormone levels or clinical thyroid outcomes in humans. The supplement rationale is mechanistically plausible but remains unvalidated by clinical evidence.
Evidence type: In vitro biochemical studies. Free DIT has been shown to exert concentration-dependent inhibitory effects on thyroid peroxidase activity in vitro. Inhibition of thyroglobulin iodination catalyzed by thyroid peroxidase was observed when free diiodotyrosine concentration was higher than 5 ΞΌM. This inhibition was competitive, suggesting that free diiodotyrosine interacts with the substrate site(s) of thyroid peroxidase. Free diiodotyrosine also competitively inhibited iodide peroxidation to Iβ. These observations were made in enzymatic assay systems and their relevance to physiological or supplemental concentrations in humans has not been established in clinical studies.
Evidence strength: In vitro only. The significance of this inhibitory activity at concentrations achievable through supplementation in vivo is unknown.
Evidence type: Preliminary/observational. Diiodotyrosine was proposed to be a new marker of leukocyte phagocytic activity in sepsis and severe infections. This observation is preliminary and has not been developed into validated clinical applications or interventional trials.
Although RIA methods have been developed for the measurement of DIT and MIT, because of limited experience, their value in clinical practice remains unknown. Early reports gave a normal mean value for DIT in the serum of normal adults of 156 ng/dL (3.6 nmol/L), with a progressive decline caused by refinement of techniques to values as low as 7 ng/dL with a range of 1 to 23 ng/dL. The normal range of 90 to 390 ng/dL for MIT is undoubtedly an overestimation. Iodotyrosine that has escaped enzymatic deiodination in the thyroid gland appears to be the principal source of DIT in serum.
There are no established dietary reference intakes, adequate intakes, or tolerable upper intake levels for MIT or DIT as isolated compounds from any major regulatory authority (including the NIH Office of Dietary Supplements, EFSA, or WHO). No human clinical trials have defined dose-response relationships for exogenous iodotyrosine supplementation in healthy subjects or in thyroid disease populations.
In the context of DEHAL1 deficiency, treatment targets the underlying iodine deficiency rather than iodotyrosine itself: the goiter and hypothyroidism of iodotyrosine deiodinase deficiency are relieved by administration of high doses of iodine. Specific dosages employed clinically in DEHAL1 deficiency management are reported in individual case reports and are not standardized in systematic reviews.
In seaweed bioavailability research, total iodine content (including organic forms) has been measured across a wide range. Total iodine concentrations in 26 seaweed samples from four species ranged from 52.4 to 1322 mg/kg of dry weight. The fraction attributable specifically to MIT and DIT within this total varies by species, with Wakame noted as containing protein-bound MIT and DIT as a proportion of its total iodine profile. No specific per-serving MIT or DIT quantities from food sources have been established in standardized reference tables at the time of writing.
High-performance liquid chromatography followed by tandem mass spectrometry (LC/MS/MS) has been used for the measurement of iodotyrosines in urine in patients with defects in iodotyrosine deiodinase, but this is a diagnostic rather than a supplementation dosage context.
Free MIT and DIT at supraphysiological concentrations have demonstrated competitive inhibition of thyroid peroxidase in vitro. Inhibition of thyroglobulin iodination catalyzed by thyroid peroxidase was observed when free diiodotyrosine concentration was higher than 5 ΞΌM; this inhibition was competitive, suggesting that free diiodotyrosine interacts with the substrate site(s) of thyroid peroxidase. Whether exogenous supplemental DIT reaches concentrations in thyroid tissue sufficient to produce this effect in vivo is unknown, as no human pharmacokinetic studies have been conducted.
Because MIT and DIT each carry one or two atoms of iodine per molecule, supplemental use carries an inherent risk of contributing to excessive iodine intake. Both iodine deficiency and iodine excess can adversely affect thyroid function. Both iodine (I) deficiency and I excess can adversely affect human health. High-dose iodine intake is known to suppress thyroid hormone synthesis through the Wolff-Chaikoff effect and can trigger or exacerbate autoimmune thyroid disorders in susceptible individuals, though this applies broadly to iodine and has not been specifically studied in the context of MIT or DIT supplementation.
3-iodotyrosine (3-IY) inhibits tyrosine hydroxylase, which catalyzes levodopa (L-DOPA) formation from tyrosine. Tyrosine hydroxylase is the rate-limiting enzyme for the synthesis of catecholamine neurotransmitters (dopamine, norepinephrine, epinephrine). Theoretical concern exists that high-dose MIT supplementation could interfere with catecholamine biosynthesis, although this has not been studied in human clinical contexts and the relevant in vivo concentrations at which this effect occurs are not established.
The bioavailability of MIT and DIT from food matrices such as seaweed is modified by how the iodine is bound within the matrix. The relative bioavailability of iodine from seaweeds varied from 18.5% to 89.0%, significantly influenced by the inorganic iodine percentage. Organically bound iodine species such as MIT and DIT may have different absorption kinetics and bioavailability compared to inorganic iodide, but comparative human bioavailability data for the isolated iodotyrosines are not available.
While this is primarily a genetic safety consideration rather than a supplementation risk, it is pharmacologically relevant: DEHAL1 defects in humans lead to severe congenital hypothyroidism not detected by neonatal screening programs, which involves the risk of mental retardation in infants. The timing for establishment of this hypothyroidism remains unknown, but environmental iodine deficiency may represent a triggering factor. Elevated urinary iodotyrosines in this population reflect pathological iodine wastage rather than beneficial supplementation.
No controlled human safety studies for supplemental MIT or DIT have been identified in the peer-reviewed literature. Absence of safety data is not equivalent to established safety. Given that both compounds are potent modulators of thyroid peroxidase activity, can contribute substantially to iodine load, and have potential interactions with catecholamine biosynthesis pathways, the safety profile of supplemental iodotyrosines in healthy or thyroid-compromised populations remains uncharacterized by clinical research.
Health conditions that Iodotyrosine may help support.
Body systems that Iodotyrosine may help support.
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