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Thyroid hormones – principally thyroxine (T4) and triiodothyronine (T3) – are master regulators of cellular metabolism in virtually every tissue in the body, acting through nuclear receptors to alter gene expression in ways that govern the rate of cell proliferation, differentiation, and metabolic activity. In skin specifically, T3 and T4 stimulate keratinocyte and fibroblast proliferation, suppress keratinocyte apoptosis, regulate glycosaminoglycan turnover in the dermis, and – in hair follicles – prolong the anagen growth phase and stimulate intrafollicular melanin synthesis. T4 is the primary secretory product of the thyroid gland but is largely a prohormone; conversion to the biologically active T3 by the deiodinase enzyme family – selenoproteins whose activity depends on adequate selenium – is the rate-limiting step between thyroid gland output and cellular effect. Understanding this conversion step is clinically important: a client can have normal circulating T4 and still have functionally insufficient T3 at tissue level if selenium is depleted – a scenario increasingly relevant in individuals on nutritionally restricted dietary patterns.

Thyroid hormones occupy an unusual position in endocrinology: they are among the most well-characterised hormonal systems in medicine, yet their peripheral actions – particularly in skin and hair – are frequently underappreciated in clinical contexts outside dermatology and endocrinology. The mechanisms by which thyroid hormones regulate skin biology are not incidental; they are foundational to the maintenance of normal epidermal turnover, dermal architecture, and hair follicle cycling. Dysfunction in either direction produces characteristic and diagnostically informative skin changes – which are covered in the Thyroid Dysfunction entity – but understanding those changes requires first understanding the normal regulatory apparatus.

The Hypothalamic-Pituitary-Thyroid Axis

Thyroid hormone production is regulated by the hypothalamic-pituitary-thyroid (HPT) axis – a classic endocrine feedback loop. The hypothalamus releases thyrotropin-releasing hormone (TRH), which signals the anterior pituitary to release thyroid-stimulating hormone (TSH). TSH acts on thyroid follicular cells to stimulate both the synthesis and secretion of thyroid hormones. As circulating T3 and T4 rise, negative feedback suppresses both TRH and TSH secretion, maintaining hormonal homeostasis within a narrow range.

The thyroid gland secretes predominantly T4 – approximately 80% of total output – with a smaller proportion of T3 secreted directly. T4 has a half-life of approximately seven days; T3 has a half-life of approximately one day. TSH is the primary clinical screening marker for thyroid function: elevated TSH indicates the pituitary is compensating for insufficient thyroid output (hypothyroidism); suppressed TSH indicates the pituitary is responding to excess thyroid hormone (hyperthyroidism). TSH alone, however, does not capture the full picture of thyroid hormone availability at tissue level – which is where peripheral conversion becomes critical.

T4, T3, and the Deiodinase System

T4 is best understood as a prohormone – biologically active in its own right but primarily serving as a circulating reservoir from which active T3 is generated in peripheral tissues. The conversion is catalysed by the deiodinase enzyme family: ^[2]

DIO1 (Type 1 deiodinase): Expressed primarily in liver, kidney, and thyroid. Converts T4 to T3 in the circulation and in peripheral tissues; also degrades inactive reverse T3 (rT3). The primary source of circulating T3 in the fed state.

DIO2 (Type 2 deiodinase): Expressed in brain, pituitary, brown adipose tissue, skeletal muscle, heart, and skin. Converts T4 to T3 locally within target tissues, providing intracellular T3 independently of circulating levels. DIO2 is the dominant source of T3 within the brain and is particularly important for ensuring adequate T3 at sites where local regulation matters.

DIO3 (Type 3 deiodinase): Inactivates both T4 (converting it to rT3) and T3 (converting it to T2). Acts as the primary terminator of thyroid hormone action; highly expressed during foetal development and in states of illness or physiological stress, where reducing T3 availability is adaptive.

The clinical significance of this system is that circulating T4 levels – the most commonly measured marker beyond TSH – do not reliably reflect T3 availability at tissue level. DIO2 activity in particular allows tissues to regulate their own T3 exposure independently of the systemic pool.

Selenium: The Rate-Limiting Cofactor

DIO1 and DIO2 are selenoproteins – their catalytic activity depends on selenocysteine at the active site, making selenium the rate-limiting cofactor for T4→T3 conversion. Selenium deficiency reduces deiodinase activity, impairing peripheral T4→T3 conversion and shifting the T4/T3 ratio: circulating T4 may be normal or even elevated whilst T3 – the biologically active hormone – falls below optimal levels. This produces a state of functional hypothyroidism at tissue level that standard TSH and T4 screening will not detect. ^[4]

The dietary context matters here. Selenium is found predominantly in animal proteins (organ meat, seafood, eggs, Brazil nuts) and its bioavailability varies significantly with soil selenium content – notably low in parts of the UK and Northern Europe. Individuals on restrictive dietary patterns, post-bariatric surgery, or in the nutritional depletion phase following rapid weight loss on GLP-1 RA medications may have suboptimal selenium intake. The deiodinase system is the mechanistic chain connecting dietary selenium status to thyroid hormone availability at tissue level, and through it, to the skin and hair consequences developed in the Thyroid Dysfunction entity.

Iodine is the second critical substrate: it is incorporated directly into T4 (four iodine atoms) and T3 (three iodine atoms) during synthesis in thyroid follicular cells. Iodine deficiency impairs T4 synthesis upstream of conversion; selenium deficiency impairs the conversion step itself. Both deficiencies can produce functionally equivalent outcomes – insufficient active T3 – through different points in the production chain.

Nuclear Receptor Mechanism

Thyroid hormones act primarily through nuclear receptors – thyroid hormone receptors TR-α and TR-β – which are ligand-activated transcription factors. T3 is the primary ligand; its affinity for both TR isoforms is approximately ten times greater than that of T4, which is why T3 is considered the biologically active form. ^[1]

Unoccupied thyroid hormone receptors typically bind to thyroid hormone response elements (TREs) in gene promoter regions in association with co-repressor complexes, actively suppressing transcription of target genes. When T3 binds, the receptor undergoes a conformational change – co-repressors are replaced by co-activator complexes – and transcription of target genes is activated. This switch from active repression to active activation means that thyroid hormone signalling is not simply additive; the difference between adequate and inadequate T3 at the nuclear receptor level is the difference between a gene being expressed and it being actively suppressed.

TR-α and TR-β have distinct tissue distributions and functions: TR-α is dominant in heart, bone, and brain; TR-β is dominant in liver, kidney, and pituitary. Both isoforms are expressed in skin and hair follicles, though the relative contributions of each to specific cutaneous functions are still being characterised. ^[1]

Thyroid Hormones in Skin

The skin is a direct target of thyroid hormone action through both keratinocytes and dermal fibroblasts. ^[5]

Keratinocyte proliferation and differentiation: T3 and T4 stimulate epidermal keratinocyte proliferation and regulate the differentiation programme that produces a functional, stratified epidermis. Adequate thyroid hormone signalling maintains normal epidermal turnover – the cycle of basal cell proliferation, upward migration, and cornification that sustains barrier function and surface texture. T3 and T4 also suppress keratinocyte apoptosis, prolonging cell survival within the differentiating layers.

Fibroblast activity: Dermal fibroblasts respond to thyroid hormone signalling with increased proliferation and extracellular matrix production – including collagen synthesis. The mechanism involves TR-mediated upregulation of growth factor receptors and downstream signalling cascades that govern cell division and matrix gene expression. Hypothyroid states reduce fibroblast activity; hyperthyroid states increase it – explaining the opposite dermal textures characteristic of each condition.

Glycosaminoglycan regulation: Thyroid hormones suppress hyaluronic acid and other glycosaminoglycan (GAG) accumulation in the dermis by downregulating the activity of dermal fibroblasts involved in GAG production. When thyroid hormone signalling is withdrawn – as in hypothyroidism – this suppression is lifted, and GAGs accumulate in the dermis. Hyaluronic acid is hygroscopic: its accumulation draws water into the dermis, producing the characteristic non-pitting, doughy oedema of myxoedema. This is not the same mechanism as inflammatory oedema; it is a direct consequence of lost hormonal regulation of GAG homeostasis. ^[6]

Thyroid Hormones in Hair Follicles

Hair follicles are among the most thyroid hormone-responsive structures in the body, expressing both TR isoforms and deiodinase enzymes (DIO2 and DIO3) that allow local intrafollicular T4→T3 conversion independently of systemic conversion. ^[7]

T4 and T3 act on hair follicle biology through several mechanisms confirmed in human hair follicle organ culture:

• Anagen prolongation: T4 suppresses TGF-β2 – a known inducer of catagen (the regression phase) – thereby extending the anagen growth phase. T3 produces a similar but less potent effect. This is the primary mechanism by which adequate thyroid hormone maintains normal hair density and growth rate. ^[7]
• Keratinocyte proliferation in the matrix: Both T3 and T4 stimulate proliferation of hair matrix keratinocytes – the rapidly dividing cells at the base of the follicle that produce the hair shaft. This effect is concentration-dependent and confirmed at physiological hormone concentrations. ^[7]
• Melanin synthesis: T3 and T4 stimulate intrafollicular melanogenesis – the production of melanin by follicular melanocytes – partly through upregulation of the melanogenic enzyme tyrosinase. The characteristic premature greying associated with thyroid dysfunction has a mechanistic basis in this regulation. ^[7]
• Stem cell activation: TR signalling in hair follicle stem cells regulates their entry into the proliferative phase at the start of each anagen cycle. Inadequate T3 at follicle level impairs stem cell responsiveness, contributing to the prolonged telogen and delayed anagen entry characteristic of hypothyroid-associated hair loss. ^[3]