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T4
Thyroxine (T4) is the principal hormone secreted by thyroid follicular cells. It contains four iodine atoms and serves largely as a circulating prohormone that can be converted in peripheral tissues to the more biologically active thyroid hormone T3.
Region-
SystemEndocrine System
Thyroxine (T4), also called tetraiodothyronine, is the major thyroid hormone secreted by the follicular cells of the thyroid gland. Its name reflects the presence of four iodine atoms within the molecule.
The thyroid secretes substantially more T4 than triiodothyronine (T3). However, T4 has lower biological activity at thyroid hormone receptors and functions importantly as a circulating prohormone from which T3 can be generated in peripheral tissues.
T4 production requires a highly specialized sequence involving dietary iodine, iodide transport into thyroid follicular cells, synthesis of thyroglobulin, oxidation and organification of iodine, coupling of iodinated tyrosine residues, storage within thyroid colloid, and TSH-stimulated release into the bloodstream.
Hormone Classification
T4 is an iodinated amino acid-derived hormone synthesized from tyrosine residues incorporated into thyroglobulin.
Unlike peptide hormones, thyroid hormones are lipid-soluble and act predominantly through intracellular nuclear receptors that regulate gene transcription.
Chemical Structure
T4 contains four iodine atoms attached to a structure derived from two iodinated tyrosine residues.
Its systematic relationship to iodine distinguishes T4 from T3, which contains three iodine atoms.
Site of T4 Production
T4 is synthesized by thyroid follicular cells, which form the epithelial walls of thyroid follicles.
These follicles represent the fundamental structural and functional units of the thyroid gland.
Thyroid Follicles
A thyroid follicle is a roughly spherical structure consisting of a layer of follicular epithelial cells surrounding a central lumen filled with colloid.
This arrangement is unusual among endocrine glands because substantial quantities of thyroid hormone precursor are stored extracellularly within the follicular lumen.
Follicular Cells
Follicular cells, also called thyrocytes, synthesize thyroglobulin and the proteins required for iodine transport and thyroid hormone synthesis.
Their morphology varies with functional activity. Relatively inactive follicles tend to have flatter epithelium, whereas actively stimulated follicular cells become more cuboidal or columnar.
Colloid
The lumen of each thyroid follicle contains colloid, a protein-rich material composed predominantly of thyroglobulin.
Iodinated thyroglobulin serves as the extracellular storage form of thyroid hormone precursors.
Thyroglobulin
Thyroglobulin is a large glycoprotein synthesized by thyroid follicular cells and secreted into the follicular lumen.
Tyrosine residues within thyroglobulin provide the molecular substrate on which T4 and T3 are synthesized.
Overview of T4 Synthesis
| Step | Process |
|---|---|
| 1 | Iodide is transported from blood into follicular cells |
| 2 | Iodide moves toward the follicular lumen |
| 3 | Iodide is oxidized by thyroid peroxidase-dependent reactions |
| 4 | Iodine is incorporated into tyrosine residues of thyroglobulin |
| 5 | Iodotyrosines undergo coupling reactions |
| 6 | T4 remains stored within iodinated thyroglobulin in colloid |
| 7 | Thyroglobulin is endocytosed into follicular cells |
| 8 | Proteolysis releases T4 and T3 |
| 9 | Thyroid hormones enter the circulation |
Iodine Requirement
Iodine is an essential dietary component required for synthesis of T4 and T3.
Dietary iodine is absorbed primarily as iodide and transported in the bloodstream, where it becomes available for uptake by thyroid follicular cells.
Sodium-Iodide Symporter
Iodide enters thyroid follicular cells across the basolateral membrane through the sodium-iodide symporter (NIS).
NIS uses the sodium gradient maintained by the sodium-potassium ATPase to concentrate iodide within follicular cells.
Iodide Trapping
The active accumulation of iodide by thyroid follicular cells is often called iodide trapping.
This process enables the thyroid gland to maintain intracellular iodide concentrations considerably greater than those in plasma.
Apical Iodide Transport
After entering follicular cells, iodide must reach the apical membrane and follicular lumen where hormone synthesis occurs on thyroglobulin.
Apical transport involves proteins including pendrin and other iodide transport mechanisms.
Thyroid Peroxidase
Thyroid peroxidase (TPO) is an enzyme located at the apical region of thyroid follicular cells.
TPO is essential for oxidation of iodide, iodination of thyroglobulin tyrosine residues, and coupling reactions that generate thyroid hormones.
Hydrogen Peroxide
Thyroid hormone synthesis requires hydrogen peroxide as an oxidizing substrate for TPO-dependent reactions.
This occurs near the apical membrane, allowing iodine chemistry to take place in association with thyroglobulin within the follicular lumen.
Organification
Organification refers to incorporation of iodine into tyrosine residues within thyroglobulin.
Iodination of tyrosine produces monoiodotyrosine and diiodotyrosine residues.
MIT and DIT
Monoiodotyrosine (MIT) contains one iodine atom, while diiodotyrosine (DIT) contains two.
These iodinated residues remain incorporated within thyroglobulin until coupling reactions generate T4 and T3.
Formation of T4
T4 is formed by coupling two DIT residues within thyroglobulin.
The newly formed T4 remains incorporated into the thyroglobulin molecule until the protein is retrieved from colloid and proteolyzed.
Formation of T3
T3 is formed primarily through coupling of one MIT residue with one DIT residue within thyroglobulin.
Although the thyroid produces both T4 and T3, much of the body's T3 is generated outside the thyroid through deiodination of circulating T4.
Iodotyrosine Coupling
| Coupling | Product |
|---|---|
| DIT + DIT | T4 |
| MIT + DIT | T3 |
Storage of T4
One of the distinctive features of thyroid physiology is the ability to store large quantities of hormone precursor extracellularly.
T4 remains incorporated within iodinated thyroglobulin in the follicular colloid, creating a hormone reserve that can support thyroid hormone secretion for an extended period.
TSH and T4 Secretion
Thyroid-stimulating hormone (TSH) from the anterior pituitary is the major trophic regulator of thyroid follicular cells.
TSH stimulates multiple steps in thyroid hormone synthesis and release, including iodide uptake, thyroglobulin processing, hormone synthesis, colloid endocytosis, and secretion.
TSH Receptor
The TSH receptor is a G protein-coupled receptor located on the basolateral membrane of thyroid follicular cells.
Binding of TSH activates intracellular signaling, particularly pathways involving cyclic AMP, that increase thyroid hormone synthesis and support follicular cell growth and function.
Endocytosis of Thyroglobulin
When stimulated by TSH, follicular cells retrieve iodinated thyroglobulin from the colloid through endocytic mechanisms.
Endocytic vesicles subsequently interact with lysosomal compartments where thyroglobulin is degraded.
Proteolysis of Thyroglobulin
Lysosomal proteases cleave thyroglobulin and release T4 and T3 from the protein backbone.
The free hormones can then cross the basolateral region of the follicular cell and enter the circulation.
Iodide Recycling
MIT and DIT residues that are not incorporated into released thyroid hormones can be deiodinated within follicular cells.
The recovered iodine is recycled for subsequent thyroid hormone synthesis.
Hypothalamic-Pituitary-Thyroid Axis
T4 secretion is regulated through the hypothalamic-pituitary-thyroid (HPT) axis.
This endocrine axis connects hypothalamic thyrotropin-releasing hormone, pituitary TSH, and thyroid hormone production.
TRH
Thyrotropin-releasing hormone (TRH) is synthesized by neurons in the hypothalamus and delivered to the anterior pituitary through the hypothalamic-hypophyseal portal circulation.
TRH stimulates pituitary thyrotrophs to synthesize and release TSH.
TSH
Thyroid-stimulating hormone, also called thyrotropin, is secreted by thyrotroph cells of the anterior pituitary.
TSH stimulates thyroid follicular cells and represents the major circulating trophic signal controlling T4 and T3 synthesis.
HPT Axis
| Level | Hormone or Structure | Action |
|---|---|---|
| Hypothalamus | TRH | Stimulates anterior pituitary thyrotrophs |
| Anterior pituitary | TSH | Stimulates thyroid follicular cells |
| Thyroid gland | T4 and T3 | Act on peripheral tissues and provide negative feedback |
Negative Feedback
Circulating thyroid hormones exert negative feedback on the hypothalamus and anterior pituitary.
Increasing thyroid hormone activity suppresses TSH secretion and reduces stimulation of the thyroid gland.
T4 in the Circulation
After secretion, most circulating T4 is reversibly bound to plasma proteins.
Only a small fraction circulates in the unbound or free T4 form.
Thyroxine-Binding Globulin
Thyroxine-binding globulin (TBG) is the principal high-affinity transport protein for thyroid hormones in blood.
Its strong binding contributes to the large circulating reservoir and relatively long biological half-life of T4.
Other T4-Binding Proteins
T4 also binds to transthyretin and albumin.
These proteins differ in affinity and capacity but collectively ensure that the overwhelming majority of circulating T4 remains protein-bound.
Major T4 Transport Proteins
| Protein | Role |
|---|---|
| Thyroxine-binding globulin | Major high-affinity thyroid hormone carrier |
| Transthyretin | Contributes to thyroid hormone transport |
| Albumin | Lower-affinity, high-capacity binding protein |
Free T4
Free T4 (FT4) refers to the small fraction of circulating T4 not bound to plasma proteins.
This unbound fraction is available for transport into tissues, metabolism, and interaction with the thyroid hormone signaling system.
Total T4
Total T4 includes both protein-bound and free hormone.
Changes in thyroid hormone-binding proteins can therefore alter total T4 concentrations without producing an equivalent change in free T4 or thyroid hormone action.
Free T4 Versus Total T4
| Measurement | What It Represents |
|---|---|
| Free T4 | Unbound circulating T4 |
| Total T4 | Free T4 plus protein-bound T4 |
T4 as a Prohormone
Although T4 has biological activity, one of its major functions is to serve as a circulating precursor for T3.
Peripheral tissues can regulate local thyroid hormone activity by converting T4 into either active T3 or inactive metabolites.
Conversion of T4 to T3
Conversion of T4 to T3 occurs through removal of an iodine atom from the outer ring of T4.
This reaction is catalyzed by enzymes called iodothyronine deiodinases.
Deiodinases
Three major deiodinase enzymes, designated D1, D2, and D3, regulate thyroid hormone activation and inactivation.
Their tissue distribution allows local control of exposure to biologically active thyroid hormone.
Type 1 Deiodinase
Type 1 deiodinase (D1) is expressed prominently in tissues including the liver, kidney, and thyroid.
It contributes to peripheral thyroid hormone metabolism and to circulating T3 production.
Type 2 Deiodinase
Type 2 deiodinase (D2) converts T4 to active T3 within several tissues, including the central nervous system, pituitary, and brown adipose tissue.
This allows tissues to regulate intracellular T3 availability even when circulating T4 remains relatively stable.
Type 3 Deiodinase
Type 3 deiodinase (D3) primarily inactivates thyroid hormones.
It can convert T4 to reverse T3 (rT3) and T3 to less active iodothyronine metabolites.
T4 Metabolic Pathways
| Pathway | Product | Functional Result |
|---|---|---|
| Outer-ring deiodination | T3 | Hormone activation |
| Inner-ring deiodination | Reverse T3 | Hormone inactivation |
Reverse T3
Reverse T3 (rT3) is an inactive metabolite generated from T4 through inner-ring deiodination.
Production of rT3 represents one pathway by which tissues can divert T4 away from formation of biologically active T3.
T4 Versus T3
| Feature | T4 | T3 |
|---|---|---|
| Common name | Thyroxine | Triiodothyronine |
| Iodine atoms | 4 | 3 |
| Major source | Direct thyroid secretion | Predominantly peripheral conversion of T4, with some direct thyroid secretion |
| Relative receptor activity | Lower | Higher |
| Circulating abundance | Higher | Lower |
| Half-life | Longer | Shorter |
| Major role | Circulating hormone reservoir and prohormone | Major active thyroid hormone at nuclear receptors |
Cellular Uptake of T4
Thyroid hormones do not simply diffuse unrestricted through all cell membranes. Specialized membrane transport proteins contribute to cellular uptake and efflux.
These include transporters such as MCT8 and other thyroid hormone transport systems.
MCT8
Monocarboxylate transporter 8 (MCT8) is an important thyroid hormone transporter encoded by the SLC16A2 gene.
It contributes to cellular transport of T4 and T3 and is particularly important for normal thyroid hormone availability in the nervous system.
Thyroid Hormone Receptors
Thyroid hormone effects are mediated primarily through nuclear thyroid hormone receptors (TRs).
T3 binds these receptors with greater functional potency than T4, which is why intracellular conversion of T4 to T3 is an important component of thyroid hormone action.
TR Alpha and TR Beta
Major thyroid hormone receptor isoforms are derived from the THRA and THRB genes.
The distribution of receptor isoforms differs among tissues, contributing to tissue-specific thyroid hormone responses.
Genomic Actions
Thyroid hormone receptors regulate transcription by interacting with specific DNA sequences known as thyroid hormone response elements.
Binding of T3 to nuclear receptors alters recruitment of transcriptional regulatory proteins and changes expression of thyroid hormone-responsive genes.
Major Physiological Effects
Because T4 serves as the principal circulating precursor for T3, its physiological importance extends across nearly every organ system.
Thyroid hormone signaling influences basal metabolic rate, thermogenesis, cardiovascular function, growth, skeletal maturation, and nervous system development.
Basal Metabolic Rate
Thyroid hormone increases energy expenditure in many tissues and contributes substantially to regulation of basal metabolic rate.
Changes in T4 availability can therefore alter oxygen consumption, substrate metabolism, and heat production.
Thermogenesis
Thyroid hormone promotes thermogenesis by increasing metabolic activity and influencing mitochondrial and cellular energy processes.
This contributes to maintenance of body temperature and adaptation to environmental conditions.
Carbohydrate Metabolism
Thyroid hormone influences intestinal glucose absorption, hepatic glucose metabolism, and peripheral carbohydrate utilization.
Excess thyroid hormone can increase metabolic turnover, while deficiency slows multiple aspects of carbohydrate metabolism.
Lipid Metabolism
Thyroid hormone strongly influences lipid synthesis, mobilization, and clearance.
It contributes to regulation of plasma cholesterol concentrations partly through effects on hepatic lipid metabolism and lipoprotein receptors.
Protein Metabolism
Physiological thyroid hormone concentrations support normal protein synthesis, growth, and tissue turnover.
Excessive thyroid hormone activity can increase protein catabolism, whereas severe deficiency can impair normal protein turnover and growth.
Cardiovascular Effects
Thyroid hormone influences heart rate, myocardial contractility, cardiac output, and peripheral vascular resistance.
Many cardiovascular effects involve increased responsiveness to catecholamines and changes in cardiac gene expression.
Thyroid Hormone and Beta-Adrenergic Responsiveness
Thyroid hormone increases expression and functional responsiveness of components of the adrenergic system in several tissues.
This contributes to features such as increased heart rate and contractility during thyroid hormone excess.
Nervous System Development
Thyroid hormone is essential for normal development of the central nervous system, particularly during fetal life and early childhood.
Adequate maternal and neonatal thyroid hormone availability is therefore critical during periods of rapid brain development.
Skeletal Growth
Thyroid hormone is necessary for normal skeletal growth and maturation.
It interacts with growth hormone and other growth-regulatory systems and is important for normal development of growth plates and bone maturation.
T4 During Pregnancy
Pregnancy produces important adaptations in thyroid physiology, including changes in thyroid hormone-binding proteins and hormone requirements.
Maternal T4 is particularly important during early fetal development before the fetal thyroid becomes fully functional.
TBG During Pregnancy
Estrogen increases hepatic production of thyroxine-binding globulin.
Consequently, total T4 concentrations normally rise during pregnancy, while physiological regulation maintains the appropriate free hormone state through adaptations of the thyroid axis.
Placental Thyroid Hormone Metabolism
The placenta expresses thyroid hormone transporters and deiodinases that regulate transfer and metabolism of maternal thyroid hormones.
This contributes to control of fetal thyroid hormone exposure during development.
T4 Half-Life
T4 has a relatively long circulating half-life, approximately seven days in euthyroid adults, although the value varies with physiological and disease states.
Extensive plasma protein binding contributes substantially to this prolonged persistence in circulation.
Metabolism and Clearance
T4 is metabolized through deiodination, conjugation, and other pathways in tissues including the liver and kidneys.
Deiodination is particularly important because it determines whether T4 is converted to active T3 or inactive metabolites.
Hypothyroidism
Hypothyroidism occurs when thyroid hormone production or action is inadequate for physiological requirements.
Primary hypothyroidism commonly produces reduced free T4 together with increased TSH because loss of thyroid hormone negative feedback increases pituitary stimulation.
Primary Hypothyroidism
In primary hypothyroidism, dysfunction occurs within the thyroid gland itself.
As T4 production decreases, reduced negative feedback allows TSH concentrations to rise in an attempt to stimulate the thyroid.
Central Hypothyroidism
Central hypothyroidism results from inadequate hypothalamic or pituitary stimulation of an otherwise potentially functional thyroid gland.
Free T4 is reduced, while TSH may be low, normal, or biologically inappropriate for the degree of thyroid hormone deficiency.
Hyperthyroidism
Hyperthyroidism refers to excessive synthesis and secretion of thyroid hormones by the thyroid gland.
In primary hyperthyroidism, increased thyroid hormone concentrations usually suppress pituitary TSH through negative feedback.
Graves Disease
Graves disease is an autoimmune cause of hyperthyroidism in which antibodies stimulate the TSH receptor.
This receptor activation increases thyroid hormone synthesis and secretion independently of normal pituitary TSH control.
Hashimoto Thyroiditis
Hashimoto thyroiditis is an autoimmune disorder characterized by immune-mediated injury to thyroid tissue and is a common cause of primary hypothyroidism.
Progressive loss of functional thyroid tissue can reduce T4 production and increase TSH through loss of negative feedback.
Iodine Deficiency
Insufficient iodine limits the thyroid gland's ability to synthesize T4 and T3.
Reduced thyroid hormone production increases TSH stimulation and can produce thyroid enlargement, or goiter.
Goiter
A goiter is enlargement of the thyroid gland and can develop in several disorders with different levels of thyroid function.
Chronic TSH stimulation caused by iodine deficiency is one mechanism through which diffuse thyroid enlargement can occur.
Thyroid Peroxidase and Autoimmunity
Thyroid peroxidase is an important autoantigen in autoimmune thyroid disease.
Antibodies against TPO are commonly detected in Hashimoto thyroiditis and may also occur in other autoimmune thyroid disorders.
TSH Measurement
Serum TSH is a highly sensitive indicator of changes in thyroid hormone feedback when the hypothalamic-pituitary axis is intact.
It is commonly interpreted together with free T4 when thyroid dysfunction is suspected.
Free T4 Measurement
Measurement of free T4 estimates the circulating fraction not bound to transport proteins.
It is particularly useful for determining the degree and pattern of thyroid hormone abnormality when interpreted alongside TSH.
Common TSH and Free T4 Patterns
| TSH | Free T4 | Typical Pattern |
|---|---|---|
| High | Low | Primary hypothyroidism |
| Low | High | Primary hyperthyroidism or thyrotoxicosis pattern |
| Low or inappropriately normal | Low | Possible central hypothyroidism |
| High | Normal | Common pattern in subclinical primary hypothyroidism |
| Low | Normal | Common pattern in subclinical hyperthyroidism |
Thyrotoxicosis
Thyrotoxicosis refers to the clinical and biochemical state caused by excessive thyroid hormone action, regardless of the source of the excess hormone.
Hyperthyroidism is one cause of thyrotoxicosis, but the terms are not completely interchangeable.
Levothyroxine
Levothyroxine is synthetic T4 and is widely used as thyroid hormone replacement therapy.
Peripheral tissues can convert administered T4 to T3 through deiodinase enzymes, allowing levothyroxine to provide substrate for tissue-specific production of active thyroid hormone.
Why T4 Is Used for Replacement
The long half-life of T4 produces relatively stable circulating concentrations with regular dosing.
Its conversion to T3 in peripheral tissues also allows individual tissues to participate in regulation of local active hormone availability.
Thyroid Hormone Synthesis Pathway
| Anatomical or Molecular Component | Role |
|---|---|
| Thyroid capillary | Delivers circulating iodide and TSH |
| Basolateral follicular membrane | Contains NIS and TSH receptors |
| Follicular cell | Synthesizes thyroglobulin and thyroid hormone machinery |
| Apical membrane | Site associated with iodide transport and TPO activity |
| Follicular colloid | Site of thyroglobulin iodination, coupling, and hormone storage |
| Endocytic and lysosomal system | Retrieves and degrades thyroglobulin to release hormones |
| Circulation | Transports T4 predominantly bound to plasma proteins |
| Peripheral tissues | Convert T4 to active T3 or inactive metabolites |
Key Features of T4
| Feature | Key Point |
|---|---|
| Full name | Thyroxine or tetraiodothyronine |
| Iodine atoms | 4 |
| Primary source | Thyroid follicular cells |
| Storage | Within iodinated thyroglobulin in thyroid colloid |
| Formation | Coupling of two DIT residues |
| Major regulator | TSH |
| Major transport protein | Thyroxine-binding globulin |
| Circulating form | Predominantly protein-bound |
| Major metabolic role | Circulating precursor for T3 |
| Activation | Outer-ring deiodination to T3 |
| Inactivation | Can be converted to reverse T3 |
| Major receptor-mediated activity | Largely through conversion to T3 and binding to nuclear thyroid hormone receptors |
| Approximate half-life | About 7 days in euthyroid adults |
| Clinical measurement | Commonly measured as free T4 with TSH |
Anatomical and Physiological Importance
T4 demonstrates the highly specialized architecture of the thyroid follicle. Unlike many endocrine hormones that are synthesized and stored entirely within endocrine cells, thyroid hormones are assembled on thyroglobulin and stored extracellularly within follicular colloid.
Follicular cells coordinate this process across two distinct membrane surfaces. The basolateral surface faces the bloodstream and contains systems responsible for iodide uptake and TSH signaling. The apical surface faces the colloid, where iodide oxidation, organification, and iodotyrosine coupling occur.
Two DIT residues are coupled within thyroglobulin to form T4. The hormone remains stored within the colloid until TSH stimulates retrieval of iodinated thyroglobulin. Lysosomal proteolysis then releases T4, allowing it to enter the bloodstream.
Once circulating, T4 is carried predominantly by plasma proteins, particularly thyroxine-binding globulin. This extensive protein binding creates a large circulating reservoir and contributes to the hormone's long half-life.
T4 subsequently reaches peripheral tissues, where deiodinase enzymes determine whether it is converted to biologically active T3 or inactive metabolites such as reverse T3. This allows individual tissues to regulate local thyroid hormone exposure rather than depending entirely on direct secretion of T3 from the thyroid gland.
Through its synthesis within thyroid follicles, extracellular storage in colloid, regulation by the HPT axis, transport by plasma proteins, and tissue-specific conversion to T3, T4 provides the major circulating reservoir through which the thyroid gland regulates metabolism, growth, development, thermogenesis, and the function of numerous organ systems.
Published on October 1, 2026
Last updated on October 1, 2026
Last updated on October 1, 2026