Zinc, Selenium, Iodine Deficiencies

Zinc Deficiency Mechanisms

• Impaired enzyme function and metalloproteins: Zinc serves as a structural or catalytic cofactor for alkaline phosphatase, lactate dehydrogenase, alcohol dehydrogenase, carbonic anhydrase, and critically, DNA polymerase and RNA polymerase. Loss of enzymatic activity directly impairs protein synthesis and cell division, with particularly severe consequences in rapidly dividing cells (intestinal epithelium, immune cells). The zinc finger motifs in transcription factors become non-functional, reducing expression of genes necessary for immune response and wound healing. This mechanism explains why zinc deficiency causes both immunodeficiency (reduced T-cell and neutrophil function) and impaired wound healing despite adequate caloric intake.

• Immune dysfunction through T-lymphocyte depletion: Zinc deficiency causes thymic atrophy and selective loss of CD4+ helper T cells through altered zinc transporter (ZIP) expression and increased apoptosis. Thymulin, a zinc-dependent hormone from thymic epithelial cells, cannot be synthesized, further impairing T-cell maturation. Neutrophil chemotaxis, phagocytosis, and oxidative burst are all impaired due to reduced expression of the zinc-dependent enzyme NADPH oxidase. IL-2 and interferon-gamma production decline, creating a state of cell-mediated immunodeficiency. This explains the characteristic pattern of recurrent infections (particularly bacterial pathogens like Staphylococcus aureus) and poor response to vaccination in zinc-deficient individuals.

• Intestinal barrier dysfunction and protein malabsorption: Zinc deficiency causes villous atrophy and reduced alkaline phosphatase activity in intestinal epithelial cells, leading to increased permeability ("leaky gut"), impaired nutrient absorption, and protein-energy malnutrition. The decreased zinc-dependent tight junction protein expression increases translocation of bacterial endotoxins, perpetuating the cycle of immune activation and further nutritional loss. Diarrhea results both from reduced enzyme activity for nutrient digestion and from increased intestinal permeability.

• Skin barrier disruption: Zinc deficiency impairs keratinocyte differentiation and collagen synthesis through reduced alkaline phosphatase and zinc finger transcription factor activity. Reduced sebaceous gland function and altered skin-associated lymphoid tissue (SALT) function compromise the physical and immunological barriers. Dermatitis develops particularly in perioral, perianal, and acral distributions due to the high turnover rate of skin cells in these areas and greater exposure to environmental irritants.

Selenium Deficiency Mechanisms

• Selenoprotein synthesis failure: Selenium is incorporated into selenoproteins through a unique translational mechanism where the UGA stop codon is read through as selenocysteine (the "21st amino acid"). Selenium deficiency reduces synthesis of critical selenoproteins including glutathione peroxidase (GPx), thioredoxin reductase (TrxR), and selenoprotein P. These enzymes are essential antioxidant defenses; without them, reactive oxygen species (ROS) accumulate intracellularly, causing direct oxidative damage to proteins, lipids, and DNA.

• Myocardial dysfunction and cardiomyopathy: Keshan disease develops in selenium-deficient individuals through accumulation of oxidative stress in cardiomyocytes, leading to inflammatory cell infiltration, myocyte necrosis, and dilated cardiomyopathy. The reduced activity of GPx allows lipid peroxides and hydrogen peroxide to accumulate, triggering apoptosis. Viral infection (particularly Coxsackievirus) is often a triggering cofactor, explaining the regional and seasonal variation in disease incidence. Arrhythmias result from both structural damage and impaired calcium handling in dysfunctional myocytes.

• Thyroid dysfunction: Selenium is required for glutathione peroxidase and thioredoxin reductase within thyroid follicular cells, protecting them from oxidative stress during hydrogen peroxide generation necessary for thyroid hormone synthesis. Selenium deficiency increases susceptibility to autoimmune thyroiditis (high selenium requirements during anti-thyroid peroxidase antibody production) and reduces conversion of T4 to active T3 by deiodinase enzymes. This creates a dual thyroid problem: impaired hormone synthesis and impaired activation.

• Immune dysregulation: Selenoproteins regulate T-cell responses and macrophage function; deficiency impairs both Th1 and Th2 responses, with relative shift toward Th1 predominance, increasing autoimmunity risk. Impaired oxidative burst in neutrophils increases infection susceptibility.

Iodine Deficiency Mechanisms

• Thyroid hormone synthesis failure: Iodine is the central structural component of thyroxine (T4) and triiodothyronine (T3), each containing 4 and 3 iodine atoms respectively. Deficiency impairs synthesis at the peroxidase-catalyzed iodination step of tyrosine residues within thyroglobulin. With inadequate iodine substrate, the thyroid gland increases mass and blood flow (compensatory hyperplasia) attempting to achieve normal hormone levels, manifesting as goiter. Eventually, with severe deficiency, thyroid hormone production fails entirely, causing hypothyroidism.

• Developmental neurotoxicity and cretinism: Iodine deficiency during pregnancy and early infancy causes severe congenital hypothyroidism, leading to endemic cretinism characterized by intellectual disability, growth retardation, and deaf-mutism. The critical mechanism involves thyroid hormone's role in myelination, synaptic development, and gene expression in the developing brain. T3 activates genes essential for neuronal migration, dendritic branching, and oligodendrocyte maturation through nuclear thyroid hormone receptors. Deficiency during the window of active neurogenesis (especially first trimester through age 2) causes irreversible developmental damage. Even subclinical iodine deficiency in older children reduces IQ by 5-10 points and impairs school performance.

• Metabolic derangement: Thyroid hormone deficiency reduces basal metabolic rate, cardiac output, GI motility, and thermogenesis through decreased expression of β-adrenergic receptors and reduced sodium-potassium ATPase activity. Myxedema, growth retardation, and delayed development result from both direct thyroid hormone deficiency and impaired growth hormone secretion.

• Goiter mechanisms: Thyroid-stimulating hormone (TSH) increases in response to low thyroid hormone, acting through the TSH receptor to stimulate thyroid follicular cell proliferation and growth. With chronic low iodine availability, this perpetuates hyperplasia and growth of the gland as it attempts to produce adequate hormone. Eventually, the gland may become so enlarged as to cause mechanical compression of the airway or esophagus.

Zinc Deficiency Causes

• Inadequate dietary intake: Occurs in populations with diets heavily dependent on plant-based foods with high phytate content (beans, legumes, whole grains) that bind zinc and reduce bioavailability; also in populations with minimal animal protein consumption. Alcoholism causes combined inadequate intake and reduced absorption. Poverty and food insecurity in developing nations represent major epidemiological drivers. Strict vegetarian/vegan diets without careful supplementation increase risk.

• Gastrointestinal malabsorption: Inflammatory bowel disease (Crohn's disease particularly) causes both reduced intestinal surface area and active secretion of zinc into the inflamed gut lumen. Short bowel syndrome after resection, celiac disease with villous atrophy, and tropical sprue all impair zinc absorption. Diarrheal diseases (infectious or inflammatory) lead to increased fecal losses exceeding dietary intake. Cystic fibrosis causes pancreatic insufficiency reducing zinc absorption.

• Long-term parenteral nutrition (TPN) without adequate supplementation: This is a high-yield cause for USMLE—zinc requirements are 2.5-4 mg/day in standard formulations, but inadequate amounts in older TPN formulas created endemic zinc deficiency in long-term home TPN patients. Peritoneal dialysis patients lose zinc through dialysate.

• Increased losses or requirements: Chronic diarrhea, whether from malabsorption, IBD, or infectious causes, increases fecal losses 5-10 fold. Hepatic cirrhosis causes both reduced dietary intake and impaired hepatic zinc metabolism. Pregnancy and lactation increase requirements 1.5-2 fold. Chronic hemolysis or wound healing from burns or trauma substantially increases requirements.

• Genetic disorders: Acrodermatitis enteropathica is an autosomal recessive disorder of the ZIP4 zinc transporter causing severe zinc malabsorption. Presents with acute dermatitis, diarrhea, and alopecia triggered by zinc-restricted diet or breast milk weaning. This is a classic board presentation.

Selenium Deficiency Causes

• Low soil selenium content: Certain geographic regions have selenium-poor soil (parts of China, New Zealand, Russia, Finland historically), creating endemic selenium deficiency. In selenium-poor regions with dietary dependence on locally grown foods, population-wide deficiency occurs. This is the primary cause of Keshan disease epidemics in rural China.

• Malabsorption syndromes: Similar to zinc, chronic diarrhea, celiac disease, cystic fibrosis, and IBD impair selenium absorption. Selenium is absorbed in the proximal small intestine; extensive small bowel resection creates severe deficiency risk.

• Long-term TPN: Selenium deficiency is well-documented in long-term TPN patients receiving formulations without adequate selenium supplementation (standard dose 40-50 μg/day). This became recognized as a cause of myocarditis and immunodeficiency in chronically ill patients receiving home TPN.

• Increased losses: Hemodialysis removes water-soluble selenium compounds; peritoneal dialysis loses selenium into the dialysate. Hyperthyroidism increases requirements due to increased selenoprotein metabolism.

Iodine Deficiency Causes

• Insufficient dietary iodine intake: Iodine deficiency is most common globally due to low iodine content of soil in many regions. Areas with iodine-poor soil include mountainous regions (Himalayas, Andes, Alps), areas where glaciation removed iodine from soil, and areas with heavy rainfall leaching iodine. Dietary sources are limited to seafood, seaweed, dairy (if cattle receive iodized salt), and eggs. Populations consuming predominantly locally-grown foods in iodine-poor regions are at highest risk.

• Increased iodine loss: Goitrogens in foods compete with iodine absorption or block thyroid hormone synthesis; these include thiocyanates (found in cruciferous vegetables: cabbage, broccoli, cassava), thionamides from certain plants, and perchlorates from contaminated groundwater. Thiocyanate has particular relevance historically (cassava root in Africa), as processing affects bioavailability of the goitrogen. Some medications (lithium, amiodarone at high doses) interfere with thyroid iodine uptake or metabolism.

• Pregnancy and lactation: Pregnancy increases iodine requirements by 50% due to increased renal clearance and increased transfer to the fetus. The developing fetus is entirely dependent on maternal thyroid hormone (and maternal iodine) until 12 weeks of gestation. Lactation loses iodine in breast milk. Inadequate supplementation during these windows causes fetal and infant deficiency.

• Premature infants: Premature infants have minimal thyroid hormone stores and cannot concentrate iodine effectively until approximately 36 weeks gestation, making them highly vulnerable to iodine deficiency in the NICU setting, particularly if receiving formula without adequate iodine supplementation.

• Cystic fibrosis and pancreatic insufficiency: Reduced fat absorption impairs absorption of iodine (which is fat-soluble) and results in excess fecal loss of thyroid hormones.

Zinc Deficiency Presentations

• Acute dermatitis (eczematous or psoriasiform): Begins with sharply demarcated erythema and scaling in perioral, perianal, and acral distributions (fingers, toes, elbows, knees, ears). Lesions often show vesiculation and crusting in acute phases and may resemble dermatitis herpetiformis or eczema. The perioral and perianal predilection is pathognomonic and relates to increased cell turnover in these thin-epithelium areas and trauma from food/feces. Pruritus is common. This is the classic presentation recognized in acrodermatitis enteropathica and distinguishes zinc deficiency from other micronutrient deficiencies on boards.

• Diarrhea and malabsorption: Chronic watery diarrhea occurs due to villous atrophy and increased intestinal permeability. May be accompanied by steatorrhea if protein malabsorption is severe. Associated weight loss is common, though the degree is variable depending on caloric intake.

• Alopecia (hair loss): Zinc deficiency impairs keratinocyte maturation in the hair follicle, leading to telogen effluvium (diffuse shedding) or frank alopecia. Hair becomes thin, brittle, and depigmented. This is particularly noticeable in patients with long-term TPN deficiency.

• Immune dysfunction manifestations: Recurrent bacterial infections (especially skin and respiratory), impaired response to vaccinations, and opportunistic infections in severe deficiency. T-cell lymphopenia with reduced CD4+ counts in laboratory assessment. Oral thrush may occur due to impaired mucosal immunity. Delayed wound healing is striking even with adequate caloric intake, with poor epithelialization and friable granulation tissue.

• Ocular manifestations: Night blindness (from impaired retinoid metabolism), photophobia, and corneal opacification can occur, though these are less common and more associated with vitamin A deficiency.

• Neuropsychiatric symptoms: Behavioral changes, depression, confusion, and apathy may occur in moderate-to-severe deficiency, though mechanism is incompletely understood (possibly related to impaired dopamine synthesis or glutamate metabolism).

• Physical examination findings: Glossitis with loss of papillae and smooth appearance of tongue; angular cheilitis (cracks at corners of mouth); nail changes including horizontal lines and ridging (leukonychia); sparse eyebrows and eyelashes; hepatosplenomegaly in advanced deficiency.

Selenium Deficiency Presentations

• Keshan disease (endemic myocarditis): Presents with acute or chronic heart failure, manifesting as dyspnea, orthopnea, peripheral edema, and cardiogenic shock in acute presentations. Exam reveals hepatomegaly, elevated JVP, peripheral edema, and signs of right heart failure. Chest X-ray shows pulmonary edema and cardiomegaly. ECG demonstrates arrhythmias (atrial fibrillation, ventricular ectopy, complete heart block), ST-T wave changes, and bradycardia. The acute presentation often follows viral infection (Coxsackievirus) in selenium-deficient populations. This is endemic in parts of China where selenium-poor soil is widespread, making it high-yield for boards as a regional disease association.

• Thyroid dysfunction: Patients develop features of hypothyroidism (fatigue, weight gain, cold intolerance, dry skin, constipation, bradycardia) with elevated TSH but may have relatively normal T4 levels. Autoimmune thyroiditis (elevated anti-TPO antibodies) is more common in selenium-deficient populations. Goiter may develop (though less dramatic than in iodine deficiency). Hair loss and nail changes similar to zinc deficiency

Zinc — sequence

• Initial test — fasting morning plasma/serum zinc: draw into a trace-element-free tube. Interpret cautiously: zinc is largely albumin-bound, so hypoalbuminemia, acute inflammation/IL-6 (which shunts zinc intracellularly), pregnancy, and corticosteroids all lower the measured value independent of body stores; conversely, a hemolyzed sample (erythrocyte zinc greatly exceeds plasma zinc) or contamination from a non-trace-element tube falsely raises it — which is why reference laboratories reject hemolyzed specimens.
• Supportive labs: a low serum alkaline phosphatase is a useful bedside clue, since ALP is a zinc metalloenzyme; it is often the first abnormality in a long-term parenteral nutrition patient.
• Confirmatory: response to a therapeutic trial of oral zinc — perioral/perianal dermatitis and diarrhea improve within days to a couple of weeks. For suspected acrodermatitis enteropathica, sequence SLC39A4 (ZIP4). Skin biopsy shows psoriasiform dermatitis with upper-epidermal pallor/necrolysis but is nonspecific (identical to necrolytic migratory erythema and pellagra).

Selenium — sequence

• Plasma selenium reflects recent intake; whole-blood or erythrocyte selenium reflects long-term status. The functional marker is plasma glutathione peroxidase activity, which plateaus once stores are replete.
• **In suspected *Keshan disease***: echocardiography is the key test, showing a dilated, globally hypokinetic ventricle with reduced ejection fraction; troponin and natriuretic peptides are elevated, and ECG shows ST-T changes, ventricular ectopy, or conduction block.

Iodine — sequence

• Urinary iodine concentration (UIC) is the WHO/UNICEF/Iodine Global Network standard. Day-to-day variability is large, so a spot UIC characterizes a population median, not an individual. By the established WHO/UNICEF/IGN epidemiologic criteria (WHO indicator document on assessment of iodine deficiency disorders), a median UIC <100 µg/L in school-age children and <150 µg/L in pregnancy define insufficient population iodine intake.
• Thyroid function: TSH elevated with low or low-normal free T4; T3 is often preserved because deficiency favors the less iodine-costly hormone. Serum thyroglobulin rises with chronic deficiency and falls with repletion.
• Neonates: universal newborn TSH screening (AAP/ATA) detects congenital hypothyroidism; an elevated proportion of neonatal TSH values above threshold is itself a population indicator of iodine deficiency.
• Imaging: ultrasound quantifies thyroid volume/goiter; radioiodine uptake is high but rarely needed.

Zinc deficiency

• First line — oral elemental zinc salts (zinc sulfate, gluconate, or acetate). WHO/UNICEF recommend 20 mg/day of elemental zinc for 10–14 days for acute childhood diarrhea (10 mg/day for infants under 6 months), which shortens duration and reduces recurrence.
• Acrodermatitis enteropathica: lifelong high-dose oral zinc (weight-based elemental zinc); dermatitis and diarrhea resolve dramatically within days — a near-diagnostic response.
• Parenteral nutrition patients: add zinc to the trace-element package and increase the dose for high-output ostomy or fistula losses, per ASPEN recommendations on parenteral trace elements.
• Avoid: chronic excess zinc, which induces enterocyte metallothionein and precipitates copper deficiency; also separate zinc from iron, calcium, and phytate-rich meals, and avoid intranasal zinc preparations.

Selenium deficiency

• Repletion with oral sodium selenite or selenomethionine, or IV selenium in parenteral nutrition (ASPEN).
• Keshan disease is managed as cardiogenic shock/acute heart failure first: diuresis, inotropic or mechanical support as needed, with selenium given concurrently — repletion prevents but does not rapidly reverse established cardiomyopathy. Surviving adults with persistent reduced ejection fraction receive full ACC/AHA/HFSA guideline-directed medical therapy: ARNI (or ACEI/ARB), beta blocker, MRA, and SGLT2 inhibitor.

Iodine deficiency

• Population-level definitive management is universal salt iodization (WHO/UNICEF); this is the intervention that eliminates endemic cretinism.
• Pregnancy and lactation: the American Thyroid Association and ACOG support a daily supplement containing 150 µg of iodine (as potassium iodide), ideally begun preconception.
• Overt hypothyroidism, including congenital: levothyroxine, started promptly in the newborn after screening confirmation (AAP/ATA) — delay costs IQ points.
• Contraindicated/cautionary: abrupt high-dose iodine in long-standing multinodular goiter can trigger Jod-Basedow iodine-induced thyrotoxicosis; conversely, iodine excess in the fetus or neonate causes Wolff-Chaikoff escape failure with goitrous hypothyroidism. ATA advises against exceeding roughly 500 µg/day of iodine in pregnancy.

Zinc deficiency

• Growth failure and hypogonadism: impaired zinc-finger transcription factor activity and reduced IGF-1 signaling produce short stature with delayed puberty and testicular atrophy — the classic Middle Eastern nutritional dwarfism description.
• Impaired wound healing and pressure injury: friable granulation tissue and wound dehiscence despite adequate calories.
• Infection: T-cell lymphopenia and defective neutrophil burst predispose to bacterial skin and respiratory infection; in severe malnutrition this drives mortality.
• Treatment complication — copper deficiency: excess supplemental zinc induces enterocyte metallothionein, trapping copper. Signals are anemia (normocytic or macrocytic) with neutropenia, and ring sideroblasts on marrow examination, plus a subacute combined degeneration-like myeloneuropathy with a normal B12 — check ceruloplasmin and serum copper.
• Intranasal zinc has caused permanent anosmia through olfactory epithelial injury.

Selenium deficiency

• Keshan disease — emergency: dilated cardiomyopathy with cardiogenic shock, malignant ventricular arrhythmias, or complete heart block. Rising troponin with a falling ejection fraction in an endemic-area or TPN patient is the signal.
• Kashin-Beck disease: endemic osteochondropathy with enlarged, painful joints and short digits from chondrocyte necrosis under oxidative stress; irreversible.
• Treatment complication — selenosis: garlic odor on the breath, brittle nails with white streaks, hair loss, GI upset, and peripheral neuropathy.

Iodine deficiency

• Endemic cretinism — irreversible intellectual disability, spastic diplegia, and deaf-mutism when deficiency spans the first trimester through age 2. Prevention is the only treatment.
• Compressive goiter — emergency when substernal extension causes stridor, positional dyspnea, or a positive Pemberton sign (facial plethora on arm elevation); requires airway assessment and surgical management.
• Myxedema coma — emergency: hypothermia, hypoventilation, hyponatremia, and depressed consciousness in profound untreated hypothyroidism.
• Treatment complications: Jod-Basedow thyrotoxicosis after iodine loading of an autonomous nodular goiter, which can escalate to thyroid storm; and neonatal goitrous hypothyroidism from maternal iodine excess.

• The zinc triad is dermatitis, diarrhea, and alopecia in a perioral, perianal, and acral distribution. In an infant, onset at weaning from breast milk points to acrodermatitis enteropathica (ZIP4/SLC39A4 defect) — breast milk contains a zinc-binding ligand that masks the defect until weaning.
• Single best next step in a long-term TPN patient with new rash, alopecia, or poor wound healing: check serum zinc — and note that a low alkaline phosphatase on the routine panel is the free clue.
• Dysgeusia/hypogeusia and impaired night vision are zinc-deficiency findings; night blindness alone is more classically vitamin A, and examiners use that as the distractor.
• Selenium = heart, iodine = brain and neck. Keshan disease is dilated cardiomyopathy in selenium-poor regions of China, often after a Coxsackievirus trigger; Kashin-Beck is the joint disease. Selenium toxicity gives ***garlic breath* and brittle nails**.
• Iodine deficiency is the leading preventable cause of intellectual disability worldwide, and the most common cause of hypothyroidism and goiter worldwide — but in the iodine-replete United States the answer is Hashimoto thyroiditis. Read the geography in the stem.
• The one association examiners test: Jod-Basedow — iodine load (contrast, amiodarone, supplementation) into a long-standing multinodular goiter causes hyperthyroidism; contrast it with Wolff-Chaikoff, the transient iodine-induced suppression that the fetus and neonate cannot escape.
• ATA/ACOG-supported pregnancy point: 150 µg/day supplemental iodine preconception through lactation; maternal T4 supplies the fetal brain until fetal thyroid function begins around the end of the first trimester.
• Common distractor to avoid: a patient on chronic zinc supplements with anemia, neutropenia, and a myelopathy resembling subacute combined degeneration is copper deficiency from zinc excess, not B12 deficiency — the B12 level will be normal.