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Prevalence of Thyroid Diseases

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1. Hypothyroidism

• Affects ~2% of women and <0.1% of men in the general population
• Annual incidence: up to 4 per 1,000 women and 1 per 1,000 men (autoimmune type)
• In iodine-replete populations, ~1-2% overall prevalence; Hashimoto thyroiditis accounts for ~85% of these cases
• Prevalence rises steeply with age; mean age at diagnosis is ~60 years
• In Down syndrome, nearly 25% have thyroid disease, most commonly primary hypothyroidism; in Turner syndrome, prevalence is ~13% but rises further by the third decade
• In pregnancy: overt hypothyroidism in 0.3-0.5% of pregnancies

• Found in 6-8% of women (up to 10% of women over 60) and 3% of men
• Postmenopausal women: prevalence approaches 20%
• Progresses to overt hypothyroidism at ~4-5% per year if TPO antibodies are positive
• In pregnancy: SCH occurs in 3-5% of pregnancies

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2. Hyperthyroidism / Thyrotoxicosis

• Prevalence: 0.2-1.3% in iodine-sufficient populations
• Europe/US: approximately 0.5-0.7%
• Australia: slightly lower, ~0.3%
• Iodine-deficient areas: higher, e.g. 2.9% in one Italian iodine-deficient village (mostly toxic nodular goiter in the elderly)
• Meta-analysis of European studies: mean prevalence 0.75%, incidence 51 per 100,000 per year
• In Asia: prevalence ranges from 0.43% to 3.9%

• Overall prevalence ~1.3% in iodine-sufficient populations
• Ethnic variation: Blacks 0.4%, Mexican Americans 0.3%, Whites 0.1%
• In iodine-deficient areas: 6-10% (mainly from toxic nodular goiters)
• Female predominance, with prevalence increasing with age

• Graves disease: 60-90% of all thyrotoxicosis cases
• Thyroiditis: ~10%
• In iodine-deficient areas: thyroidal autonomy (toxic nodular goiter) predominates

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3. Autoimmune Thyroid Disease (AITD)

• Overall prevalence: ~5% of the general population
• Thyroid autoantibodies (TPO-Ab or TG-Ab) detected in 10-20% of the general population, even in euthyroid individuals
• Strong female predominance (approximately 5:1 to 10:1, female:male)
• ~50% of adult Turner syndrome patients have anti-TPO or anti-TG autoantibodies

• Most common cause of hyperthyroidism in iodine-sufficient regions (60-90% of thyrotoxicosis)
• About ~50 million prevalent cases of Hashimoto thyroiditis estimated in the major world markets (US, EU4+UK, Japan) in 2024

• Occurs in 5-9% of postpartum women, usually self-limiting
• 20-40% of affected women develop permanent hypothyroidism
• The American Thyroid Association recommends TSH/free T4/TPO screening for women with postpartum depression

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4. Goiter (Thyroid Enlargement)

• Endemic goiter: defined when goiter affects >5% of the population in a region; caused by iodine deficiency. This is the most common cause of thyroid disease worldwide when considering all countries.
• Palpable goiter: found in ~15% of persons in an iodine-replete region (Whickham Survey)
• Visible goiter: ~7% in the Whickham Survey
• In iodine-deficient areas: much higher - one study in Pomerania (before iodine supplementation) found goiter in 35.9% of individuals without known thyroid disease
• With high-resolution ultrasound: goiter/thyroid enlargement detected in 19-67% of imaged individuals
• Higher frequencies in women and the elderly
• Iodine supplementation programs have significantly reduced endemic goiter globally

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5. Thyroid Nodules

• Palpable thyroid nodules: ~5% of adults (varies considerably worldwide)
• Lifetime risk of developing a thyroid nodule: 5-10% (US Framingham study)
• Ultrasound-detected nodules: found in up to 50% of individuals aged >50 years, reflecting the high prevalence of incidentalomas
• More common in women, the elderly, and iodine-deficient populations
• ~10-15% of nodules biopsied with FNA are found to be malignant

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6. Thyroid Cancer

• Accounts for only ~1% of all new malignant diagnoses globally, but incidence is rising (mainly due to increased detection of small papillary cancers)
• Prevalence in nodular thyroid disease: 0.4-1.8% for medullary thyroid cancer; overall malignancy rate in nodules is much higher depending on ultrasound features

• PTC is the most common in children as well (~85% of childhood thyroid cancer)
• Incidence of thyroid cancer has been rising globally since the 1990s, primarily due to detection of small (<1 cm) papillary cancers; cancer mortality from advanced-stage disease has also been rising in parallel

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Summary Table

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• Iodine status is the single most important environmental determinant of thyroid disease burden globally.
• In iodine-sufficient countries, autoimmune disease (Hashimoto/Graves) dominates.
• In iodine-deficient regions, endemic goiter and toxic nodular goiter are most prevalent.
• Women are consistently 5-10 times more affected than men across almost all thyroid conditions.
• Subclinical forms of hypo- and hyperthyroidism are far more prevalent than overt disease.

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Thyroid Swellings - Classification and Most Common Age Groups

1. Morphology - diffuse vs. nodular
2. Functional state - toxic (hyperthyroid), nontoxic (euthyroid), or hypothyroid
3. Cause - inflammatory, autoimmune, neoplastic, physiological, or iodine-related
4. Number of nodules - solitary vs. multinodular

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1. Simple/Physiological Diffuse Goiter

• Age: Adolescents and young adults (10-25 years), especially at puberty and during pregnancy
• Female predominance
• Caused by transient increased thyroid hormone demand during rapid growth, puberty, or pregnancy
• Typically euthyroid; often regresses spontaneously
• Referred to as "juvenile goiter" in teenagers

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2. Endemic Goiter (Diffuse, Iodine-Deficient)

• Age: All age groups, but incidence increases with age due to cumulative environmental exposure
• Female predominance (2-3x higher than males)
• Affects populations in iodine-deficient regions (mountainous areas, inland territories)
• In severely iodine-deficient areas, a palpable goiter can be found in 40-90% of individuals
• Over 2 billion people worldwide are exposed to iodine-deficient diets

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3. Sporadic Multinodular Goiter (MNG / Nontoxic)

• Age: Middle-aged and older adults (40-60+ years); incidence increases with age
• Most common cause of nontoxic goiter in iodine-rich countries (~5% incidence)
• Strong female predominance
• Cause largely unknown in iodine-sufficient populations
• The incidence parallels the age-related rise in thyroid nodules generally
• Clinically: anterior neck mass, often bilateral, may cause compressive symptoms (dysphagia, hoarseness, stridor) as it grows

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4. Toxic Multinodular Goiter (Plummer Disease)

• Age: Adults over 50 years, especially in iodine-deficient regions
• Most common cause of hyperthyroidism in the elderly and in iodine-deficient populations (second most common overall in the US after Graves' disease)
• Female predominance (~5:1 female:male ratio)
• Constitutively activating TSH receptor mutations in autonomous nodules

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5. Graves' Disease (Diffuse Toxic Goiter)

• Age: Young adults, 20-40 years (peak onset); can occur in children and elderly too
• Female predominance: 7-8:1 female:male ratio - the most female-dominant of all thyroid diseases
• Most common cause of hyperthyroidism in iodine-sufficient countries (60-90% of thyrotoxicosis)
• Presents with diffuse, smooth, firm goiter + features of hyperthyroidism + exophthalmos (in ~25-30%)
• Incidence: ~30 cases per 100,000 persons per year

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6. Hashimoto Thyroiditis (Chronic Autoimmune Thyroiditis)

• Age: 30-50 years most commonly; can also occur in children and teenagers (most common thyroid disease in pediatric age group, typically >10 years)
• Female predominance (4-10:1)
• Causes a firm, diffuse, non-tender goiter initially; gland atrophies over time
• In children: strong female predominance (87.7%), most >10 years and pubertal at diagnosis; neck swelling present in ~23%
• Most common cause of hypothyroidism in iodine-replete populations

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7. Subacute (De Quervain) Thyroiditis

• Age: 30-50 years, but can occur at any age
• Follows a viral upper respiratory tract infection
• Female predominance (~4:1)
• Presents with painful, tender, firm goiter + fever + raised ESR
• Usually self-limiting; ~10% develop permanent hypothyroidism

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8. Solitary Toxic Adenoma

• Age: Median 50-60 years; female predominance (mild)
• A single autonomously functioning "hot" nodule
• Less common than MNG or Graves'; rarely malignant

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9. Thyroid Cancer (Malignant Swelling)

• Overall thyroid cancer: most common in ages 35-39 or >70 (UK data)
• Women are 2-3x more likely to develop thyroid cancer than men
• In children, papillary carcinoma accounts for ~85% of thyroid cancers

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10. Substernal Goiter

• Age: >60 years predominates (60% of cases occur in patients over 60)
• A goiter extending inferiorly into the mediastinum through the thoracic inlet
• Incidence: ~0.02% general population
• May present with compressive symptoms - SVC syndrome, tracheal deviation, stridor
• Almost always an extension of a long-standing cervical goiter

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Summary Table: Thyroid Swellings by Age Group

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• The 3rd and 4th decades (21-40 years) see the overall highest frequency of thyroid swellings presenting clinically, with females accounting for the large majority (F:M ratios range from 2.8:1 to 10:1 depending on the series)
• 82% of thyroid swellings are euthyroid at presentation (colloid nodule and MNG being most common on ultrasound)
• Colloid nodule is the most common benign finding on imaging; papillary carcinoma is the most common malignant cause
• Any rapidly growing thyroid mass in an elderly patient should raise suspicion for anaplastic carcinoma or lymphoma

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Normal Thyroid Profile Reference Ranges

Important note: Exact reference ranges are assay-specific and laboratory-specific. The values below represent the most widely cited ranges from authoritative sources. Always interpret results against your own laboratory's reference interval.

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1. TSH (Thyroid-Stimulating Hormone) - The Master Screening Test

• Inverse log-linear relationship with free T4 - small changes in T4 cause large shifts in TSH, making it the most sensitive screening test
• Third-generation assays can detect down to 0.005 mIU/L
• TSH <0.1 mIU/L = subclinical hyperthyroidism territory
• TSH <0.01 mIU/L = frank Graves' thyrotoxicosis
• TSH 4-10 mIU/L = subclinical hypothyroidism range
• TSH >10 mIU/L = overt hypothyroidism (symptomatic)
• Suppressed by: glucocorticoids, dopamine, octreotide, severe illness

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2. Free T4 (fT4 / Free Thyroxine) - Primary Hormone Assessment

• Only the free (unbound) fraction is biologically active (~0.03% of total T4)
• More reliable than total T4 in settings of altered protein binding (pregnancy, estrogen therapy, liver disease, nephrotic syndrome)
• Low fT4 + high TSH = overt primary hypothyroidism
• High fT4 + low TSH = overt hyperthyroidism
• Normal fT4 + high TSH = subclinical hypothyroidism
• Normal fT4 + low TSH = subclinical hyperthyroidism

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3. Free T3 (fT3 / Free Triiodothyronine) - Most Biologically Active Hormone

• Most potent thyroid hormone (3-4x more active than T4)
• ~80% of T3 is produced by peripheral conversion from T4 (liver, kidneys)
• Free T3 measurement is technically challenging (very low concentrations)
• Most useful in T3 thyrotoxicosis: TSH suppressed, fT4 normal, fT3 elevated
• Also rises early in hyperthyroidism before total T4 becomes abnormal

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4. Total T4 (Thyroxine)

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5. Total T3 (Triiodothyronine)

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6. T3 Resin Uptake (T3-RU) and Free Thyroxine Index (FTI)

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7. Radioactive Iodine Uptake (RAIU)

• Elevated in Graves' disease and iodine deficiency
• Low/absent in thyroiditis, factitious thyrotoxicosis, excess iodine
• Not a routine screening test; used primarily in hyperthyroid workup

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8. Thyroid Antibodies

• Anti-TPO antibodies found in 10-20% of general population (even in euthyroid individuals)
• Positive anti-TPO in subclinical hypothyroidism = ~5% annual risk of progression to overt hypothyroidism
• Calcitonin is a marker for medullary thyroid cancer (normal: <10 pg/mL in most labs)
• Thyroglobulin (Tg): used as tumor marker post-thyroidectomy for differentiated thyroid cancer (should be undetectable after total thyroidectomy)

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9. Age- and Condition-Specific Variations

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Quick Interpretation Summary