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Hypothyroidism
Hypothyroidism is an endocrine disorder caused by insufficient thyroid hormone production or activity. It commonly results from primary thyroid gland dysfunction and produces widespread effects on metabolism, cardiovascular function, thermoregulation, gastrointestinal activity, the nervous system, skin, muscles, and other tissues.
RegionHead and Neck
SystemEndocrine System
Hypothyroidism is a clinical and biochemical state caused by insufficient production or action of thyroid hormones. Because thyroid hormones influence metabolic activity in tissues throughout the body, deficiency can affect cardiovascular function, thermoregulation, gastrointestinal motility, nervous system activity, skeletal muscle, skin, reproductive function, and energy metabolism.
The thyroid gland normally produces predominantly thyroxine (T4) and smaller quantities of triiodothyronine (T3). Much of the biologically active T3 in peripheral tissues is produced through conversion of T4 to T3. Thyroid hormone synthesis and secretion are regulated principally through the hypothalamic-pituitary-thyroid axis.
Hypothyroidism is classified according to the anatomical level of dysfunction. Primary hypothyroidism results from disease of the thyroid gland itself and is the most common form. Central hypothyroidism results from inadequate stimulation of an otherwise potentially functional thyroid because of hypothalamic or pituitary disease.
Thyroid Gland
The thyroid gland is an endocrine organ located in the anterior neck, typically extending across the lower cervical region anterior and lateral to the upper trachea.
It consists of right and left lobes connected by an isthmus. A pyramidal lobe may also be present.
Thyroid Follicles
The functional structural units of the thyroid are the thyroid follicles.
Each follicle consists of a layer of follicular epithelial cells surrounding a central lumen filled with colloid.
Follicular Cells
Follicular cells synthesize thyroglobulin and participate in iodide uptake, thyroid hormone synthesis, storage, and secretion.
The morphology of these cells varies according to their functional activity.
Colloid
The follicular lumen contains colloid, which consists largely of thyroglobulin.
The thyroid is unusual among endocrine glands because substantial quantities of hormone precursor are stored extracellularly within follicular colloid.
Thyroid Hormones
The principal thyroid hormones are:
- Thyroxine (T4)
- Triiodothyronine (T3)
T4 is secreted in greater quantities by the thyroid gland, while T3 has greater biological activity at thyroid hormone receptors.
Thyroid Hormone Synthesis
Thyroid hormone synthesis requires coordinated uptake of iodide, synthesis of thyroglobulin, oxidation and organification of iodine, coupling of iodinated tyrosine residues, storage within colloid, and subsequent release of T3 and T4.
Iodide Uptake
Circulating iodide is transported into thyroid follicular cells by the sodium-iodide symporter located on the basolateral membrane.
This allows the thyroid to concentrate iodide relative to its concentration in plasma.
Thyroglobulin
Thyroglobulin is a large glycoprotein synthesized by thyroid follicular cells and secreted into the follicular lumen.
Tyrosine residues within thyroglobulin provide the molecular framework on which thyroid hormones are synthesized.
Thyroid Peroxidase
Thyroid peroxidase (TPO) is an important enzyme located near the apical surface of thyroid follicular cells.
It participates in oxidation of iodide, iodination of tyrosine residues within thyroglobulin, and coupling reactions required for formation of thyroid hormones.
Organification
Iodine is incorporated into tyrosine residues within thyroglobulin through a process called organification.
This produces monoiodotyrosine and diiodotyrosine residues.
Formation of T3 and T4
Coupling of iodinated tyrosine residues produces thyroid hormones.
| Coupling Reaction | Product |
|---|---|
| MIT + DIT | T3 |
| DIT + DIT | T4 |
Release of Thyroid Hormones
Thyroglobulin containing stored thyroid hormones is taken back into follicular cells and processed within lysosomal compartments.
T3 and T4 are released from thyroglobulin and subsequently enter the circulation.
Peripheral Conversion of T4
Much of the body's T3 is produced outside the thyroid gland through enzymatic removal of an iodine atom from T4.
This conversion occurs in several tissues and allows local regulation of thyroid hormone activity.
Hypothalamic-Pituitary-Thyroid Axis
Thyroid function is controlled primarily through the hypothalamic-pituitary-thyroid (HPT) axis.
The hypothalamus secretes thyrotropin-releasing hormone, which stimulates the anterior pituitary to release thyroid-stimulating hormone. TSH then stimulates thyroid follicular cells.
Normal HPT Axis
The regulatory pathway can be summarized as:
Hypothalamus → TRH → Anterior pituitary → TSH → Thyroid gland → T4 and T3
Thyroid-Stimulating Hormone
Thyroid-stimulating hormone (TSH), also called thyrotropin, stimulates multiple aspects of thyroid follicular cell function.
Its actions include promotion of iodide uptake, hormone synthesis, hormone release, and maintenance of thyroid gland growth and activity.
Negative Feedback
Circulating thyroid hormones exert negative feedback on the hypothalamus and anterior pituitary.
When thyroid hormone concentrations fall because of primary thyroid dysfunction, this feedback decreases and TSH secretion generally increases.
Primary Hypothyroidism
Primary hypothyroidism results from dysfunction of the thyroid gland itself.
The thyroid cannot produce sufficient hormone despite stimulation by TSH.
Hormonal Pattern in Primary Hypothyroidism
The typical pattern in overt primary hypothyroidism is:
Low free T4 + elevated TSH
The increased TSH represents the pituitary response to inadequate thyroid hormone-mediated negative feedback.
Causes of Primary Hypothyroidism
Primary hypothyroidism can result from several processes affecting thyroid tissue or thyroid hormone synthesis.
- Autoimmune thyroid disease
- Iodine deficiency
- Thyroid surgery
- Radioiodine treatment
- Selected medications
- Congenital abnormalities
- Inflammatory or infiltrative thyroid disorders
Hashimoto Thyroiditis
Hashimoto thyroiditis, also called chronic autoimmune thyroiditis, is an important cause of primary hypothyroidism.
Immune-mediated injury progressively damages thyroid follicular cells and can eventually reduce the gland's ability to produce sufficient thyroid hormone.
Thyroid Autoantibodies
Autoimmune thyroid disease is commonly associated with antibodies against thyroid antigens.
Important antibodies include:
- Thyroid peroxidase antibodies
- Thyroglobulin antibodies
Histology of Hashimoto Thyroiditis
Hashimoto thyroiditis is characterized histologically by lymphocytic infiltration of thyroid tissue, formation of lymphoid follicles in many cases, progressive destruction of thyroid follicles, and characteristic epithelial changes.
Hürthle Cells
Follicular epithelial cells in Hashimoto thyroiditis may undergo transformation into Hürthle cells.
These cells have abundant granular eosinophilic cytoplasm because of accumulation of mitochondria.
Thyroid Size in Autoimmune Disease
The thyroid can initially become enlarged because of inflammation and TSH-mediated stimulation.
With longstanding destructive disease, the gland may become smaller and atrophic.
Iodine Deficiency
Iodine is required for synthesis of both T3 and T4.
Insufficient iodine availability reduces thyroid hormone synthesis and can produce hypothyroidism when deficiency is sufficiently severe.
Goiter in Iodine Deficiency
Reduced thyroid hormone synthesis decreases negative feedback on the pituitary, increasing TSH stimulation.
Persistent TSH stimulation can cause thyroid follicular hypertrophy and hyperplasia, producing a goiter.
Postoperative Hypothyroidism
Removal of sufficient thyroid tissue during thyroid surgery can reduce the gland's capacity to produce thyroid hormone.
The risk depends on the extent of thyroid removal and the functional capacity of remaining tissue.
Radioiodine-Associated Hypothyroidism
Radioactive iodine can be used to destroy thyroid tissue in selected thyroid disorders.
Loss of sufficient functional follicular tissue can subsequently produce permanent hypothyroidism.
Drug-Associated Hypothyroidism
Several medications can interfere with thyroid hormone synthesis, secretion, metabolism, or immune regulation and can contribute to hypothyroidism in susceptible individuals.
Congenital Hypothyroidism
Congenital hypothyroidism is thyroid hormone deficiency present at or shortly after birth.
Causes include abnormal thyroid development, defects in thyroid hormone synthesis, iodine-related abnormalities, and central disorders.
Thyroid Dysgenesis
Abnormal embryological development of the thyroid gland is an important cause of congenital primary hypothyroidism.
The gland may be absent, incompletely developed, or located ectopically because of abnormal migration during development.
Importance of Thyroid Hormone in Development
Thyroid hormone is particularly important for normal growth and development of the central nervous system during fetal and early postnatal life.
Severe untreated deficiency during critical developmental periods can cause irreversible neurological impairment.
Central Hypothyroidism
Central hypothyroidism results from inadequate stimulation of the thyroid gland because of disease involving the pituitary, hypothalamus, or their regulatory pathways.
The thyroid gland itself may retain the capacity to synthesize hormone if adequately stimulated.
Secondary Hypothyroidism
Secondary hypothyroidism results from inadequate biologically effective TSH secretion from the anterior pituitary.
This can occur in association with pituitary tumors, surgery, radiation, infarction, inflammatory disease, or other causes of pituitary dysfunction.
Tertiary Hypothyroidism
Tertiary hypothyroidism refers to hypothalamic dysfunction causing inadequate TRH stimulation of the pituitary-thyroid axis.
In clinical practice, secondary and tertiary forms are commonly grouped under the broader term central hypothyroidism.
Primary Versus Central Hypothyroidism
| Feature | Primary Hypothyroidism | Central Hypothyroidism |
|---|---|---|
| Primary site | Thyroid gland | Pituitary or hypothalamus |
| Free T4 | Low in overt disease | Low |
| TSH | Usually elevated | Low, normal, or sometimes mildly elevated but biologically inappropriate |
| Thyroid response | Unable to produce adequate hormone | Insufficient central stimulation |
Subclinical Hypothyroidism
Subclinical hypothyroidism is characterized biochemically by an elevated TSH concentration with free T4 remaining within the reference range.
It often represents mild or early thyroid failure, although the clinical course varies.
Metabolic Effects of Thyroid Hormone Deficiency
Thyroid hormones normally increase metabolic activity in many tissues.
Deficiency therefore produces a generalized reduction in metabolic processes and contributes to many characteristic manifestations of hypothyroidism.
Basal Metabolic Rate
Reduced thyroid hormone activity decreases basal metabolic rate.
Lower energy expenditure contributes to fatigue, cold intolerance, and a tendency toward weight gain.
Cold Intolerance
Thyroid hormones contribute to heat production through their effects on cellular metabolism.
Reduced thermogenesis in hypothyroidism can produce pronounced cold intolerance.
Weight Gain
Weight gain can occur because of reduced energy expenditure and accumulation of extracellular fluid.
The degree of weight gain varies and is not necessarily proportional to the severity of thyroid hormone deficiency.
Fatigue
Fatigue and reduced energy are common manifestations of hypothyroidism.
They reflect reduced metabolic activity and effects on multiple organ systems.
Cardiovascular Effects
Thyroid hormones influence heart rate, myocardial contractility, vascular resistance, and cardiovascular responsiveness to catecholamines.
Deficiency can therefore produce characteristic cardiovascular changes.
Bradycardia
Reduced thyroid hormone activity can decrease sinoatrial node activity and adrenergic responsiveness, producing a slower resting heart rate.
Cardiac Output
Reduced heart rate and myocardial contractility can decrease cardiac output.
Exercise capacity may also decline.
Systemic Vascular Resistance
Systemic vascular resistance can increase in hypothyroidism because of reduced metabolic vasodilation and altered vascular function.
Pericardial Effusion
Severe or longstanding hypothyroidism can be associated with accumulation of fluid within the pericardial cavity.
Similar serous effusions may occasionally occur in other body cavities.
Cardiovascular Features
| Variable | Typical Effect |
|---|---|
| Heart rate | Decreased |
| Myocardial contractility | Reduced |
| Cardiac output | Reduced |
| Systemic vascular resistance | Often increased |
| Pericardial fluid | May increase in severe disease |
Effects on Lipid Metabolism
Thyroid hormone deficiency can reduce hepatic clearance of cholesterol-containing lipoproteins.
Elevated total cholesterol and low-density lipoprotein cholesterol can therefore occur in untreated hypothyroidism.
Gastrointestinal Effects
Reduced thyroid hormone activity can slow gastrointestinal motility.
This commonly contributes to constipation.
Nervous System Effects
Thyroid hormone deficiency can affect cognition, mood, peripheral nerve function, and neuromuscular activity.
Patients may experience slowed thinking, impaired concentration, somnolence, or other neurological manifestations.
Cognitive Slowing
Severe hypothyroidism can produce slowing of mental processing and reduced alertness.
These abnormalities generally improve when the underlying thyroid hormone deficiency is corrected, although developmental thyroid hormone deficiency has different implications.
Peripheral Nerve Effects
Peripheral neuropathic symptoms can occur in longstanding hypothyroidism.
Tissue changes can also contribute to entrapment neuropathies such as carpal tunnel syndrome.
Musculoskeletal Effects
Hypothyroidism can produce muscle stiffness, cramps, weakness, and discomfort.
Deep tendon reflexes may demonstrate delayed relaxation.
Delayed Reflex Relaxation
Delayed relaxation of deep tendon reflexes is a classic physical finding associated with hypothyroidism.
It reflects altered muscle contraction and relaxation physiology in the setting of thyroid hormone deficiency.
Skin Changes
The skin in hypothyroidism can become cool, dry, and coarse.
Reduced cutaneous blood flow, altered glandular secretion, and changes in dermal connective tissue contribute to these findings.
Hair Changes
Hair may become coarse, dry, or brittle, and diffuse hair loss can occur.
Loss or thinning of the lateral portions of the eyebrows can occur in longstanding disease.
Myxedema
Myxedema refers to characteristic tissue changes associated with severe hypothyroidism, particularly accumulation of hydrophilic glycosaminoglycans within the interstitial tissues.
These molecules attract water and contribute to nonpitting thickening and swelling of the skin and subcutaneous tissues.
Facial Changes
Severe hypothyroidism can produce a characteristic puffy appearance of the face, including swelling around the eyes.
These changes are related partly to accumulation of interstitial material and fluid.
Voice Changes
Infiltration and swelling of tissues involving the upper airway and laryngeal structures can contribute to a hoarse or deepened voice.
Reproductive Effects
Thyroid hormone deficiency can alter reproductive endocrine function.
Menstrual irregularities, impaired fertility, and changes in reproductive function can occur depending on the severity and duration of disease.
Prolactin and Hypothyroidism
In primary hypothyroidism, increased hypothalamic TRH activity can stimulate prolactin secretion in some patients.
This provides one mechanism by which severe primary hypothyroidism can affect reproductive function.
Clinical Features
| Feature | Underlying Effect |
|---|---|
| Fatigue | Reduced metabolic activity |
| Cold intolerance | Reduced thermogenesis |
| Weight gain | Reduced energy expenditure and fluid accumulation |
| Bradycardia | Reduced thyroid-mediated cardiac stimulation |
| Constipation | Reduced gastrointestinal motility |
| Dry skin | Altered cutaneous metabolism and glandular activity |
| Slowed cognition | Reduced central nervous system activity |
| Delayed reflex relaxation | Altered neuromuscular physiology |
| Myxedema | Interstitial glycosaminoglycan accumulation |
Goiter
A goiter is enlargement of the thyroid gland and can occur in some forms of hypothyroidism.
Persistent TSH stimulation can promote follicular hypertrophy and hyperplasia when thyroid hormone synthesis is impaired but thyroid tissue remains responsive.
Why Hypothyroidism Does Not Always Cause Goiter
The anatomical appearance of the thyroid depends on the cause of hypothyroidism.
A gland exposed to chronic TSH stimulation may enlarge, whereas extensive autoimmune destruction, surgical removal, radioiodine ablation, or developmental absence can produce a small or absent gland.
Diagnosis
Laboratory assessment is central to the diagnosis of hypothyroidism.
The most important measurements generally include serum TSH and free T4, interpreted according to the suspected anatomical level of dysfunction.
TSH Measurement
TSH is particularly useful for detecting primary thyroid dysfunction because the pituitary responds sensitively to changes in circulating thyroid hormone concentrations.
An elevated TSH with low free T4 strongly supports overt primary hypothyroidism.
Free T4
Measurement of free T4 provides information about circulating biologically available thyroxine.
It is especially important when central hypothyroidism is suspected because TSH alone may be misleading.
Typical Laboratory Patterns
| Condition | TSH | Free T4 |
|---|---|---|
| Overt primary hypothyroidism | Elevated | Low |
| Subclinical primary hypothyroidism | Elevated | Within reference range |
| Central hypothyroidism | Low or inappropriately normal, occasionally mildly elevated | Low |
Thyroid Peroxidase Antibodies
Measurement of TPO antibodies can help identify autoimmune thyroid disease as the cause of primary hypothyroidism.
Antibody positivity indicates thyroid autoimmunity but must be interpreted with thyroid function tests and clinical findings.
Thyroid Imaging
Imaging is not required to diagnose most cases of uncomplicated primary hypothyroidism.
Ultrasound can provide anatomical information when structural thyroid abnormalities, nodules, asymmetry, or enlargement require evaluation.
Myxedema Coma
Myxedema coma is a rare, severe, life-threatening manifestation of profound hypothyroidism.
Despite its name, affected patients are not necessarily comatose. The condition represents severe decompensated thyroid hormone deficiency with failure of multiple physiological systems.
Precipitating Factors
Severe decompensation can be triggered by physiological stress in a person with longstanding or inadequately treated hypothyroidism.
Potential precipitating factors include:
- Infection
- Cold exposure
- Surgery
- Trauma
- Selected medications that depress central nervous system function
- Discontinuation or inadequate thyroid hormone replacement
Features of Myxedema Coma
Potential manifestations include:
- Altered mental status
- Hypothermia
- Bradycardia
- Hypotension
- Hypoventilation
- Hyponatremia
- Hypoglycemia
- Severe generalized metabolic slowing
Respiratory Effects in Severe Hypothyroidism
Severe hypothyroidism can reduce ventilatory drive and impair respiratory muscle function.
These abnormalities can contribute to hypoventilation during myxedema coma.
Hyponatremia
Severe hypothyroidism can impair free-water excretion and contribute to hyponatremia.
The mechanism can involve altered renal hemodynamics and inappropriate retention of water relative to sodium.
Hypothermia
Profound reduction in metabolic heat production can produce significant hypothermia in severe hypothyroidism.
Treatment Principles
Treatment of hypothyroidism is based on replacement of deficient thyroid hormone.
Levothyroxine, a synthetic form of T4, is commonly used for long-term replacement therapy.
Levothyroxine
Administered T4 provides circulating thyroid hormone that can be converted to T3 in peripheral tissues.
The dose is individualized according to factors such as the patient's age, body size, residual thyroid function, cardiovascular status, pregnancy status, and laboratory response.
Monitoring Primary Hypothyroidism
In primary hypothyroidism, serum TSH is generally an important marker for adjusting long-term thyroid hormone replacement.
Because the HPT axis requires time to reach a new steady state after a dose change, laboratory reassessment is performed after an appropriate interval.
Monitoring Central Hypothyroidism
TSH cannot be relied upon in the same way for central hypothyroidism because pituitary or hypothalamic dysfunction is responsible for the disorder.
Free T4 and the clinical context are therefore particularly important for monitoring replacement.
Hypothyroidism Versus Hyperthyroidism
| Feature | Hypothyroidism | Hyperthyroidism |
|---|---|---|
| Thyroid hormone activity | Reduced | Increased |
| Metabolic rate | Reduced | Increased |
| Temperature tolerance | Cold intolerance | Heat intolerance |
| Heart rate | Often decreased | Often increased |
| Weight tendency | Gain | Loss |
| Bowel activity | Reduced | Increased |
| Neuromuscular activity | Slowed | Often increased |
Relationship to Thyroid Anatomy
Hypothyroidism illustrates how structural abnormalities at different levels of the endocrine axis can produce a similar final state of thyroid hormone deficiency.
Destruction or absence of thyroid follicular tissue causes primary disease, while lesions involving the pituitary or hypothalamus reduce the trophic signals required for normal thyroid activity.
Key Features of Hypothyroidism
| Feature | Key Point |
|---|---|
| Primary hormones affected | T4 and T3 |
| Primary endocrine organ | Thyroid gland |
| Functional unit | Thyroid follicle |
| Hormone-producing cells | Follicular cells |
| Major regulatory hormone | TSH |
| Common primary mechanism | Autoimmune thyroid destruction |
| Overt primary laboratory pattern | High TSH with low free T4 |
| Central laboratory pattern | Low free T4 with an inappropriately low, normal, or occasionally mildly elevated TSH |
| Characteristic metabolic effect | Reduced metabolic activity |
| Severe decompensation | Myxedema coma |
Clinical Associations
| Finding or Disorder | Relationship to Hypothyroidism |
|---|---|
| Hashimoto thyroiditis | Important autoimmune cause of primary hypothyroidism |
| Goiter | Can result from persistent TSH stimulation |
| Hypercholesterolemia | Can result partly from reduced lipoprotein clearance |
| Myxedema | Interstitial accumulation of hydrophilic glycosaminoglycans |
| Congenital hypothyroidism | Can impair normal neurological development if untreated |
| Myxedema coma | Severe decompensated hypothyroidism |
Anatomical and Physiological Importance
Hypothyroidism demonstrates the extensive physiological influence of the thyroid gland. Thyroid follicles occupy a relatively small anatomical region in the anterior neck, yet the hormones they produce regulate metabolic activity throughout much of the body. Loss of adequate thyroid hormone therefore produces systemic effects rather than abnormalities confined to the neck.
The disorder also illustrates the importance of endocrine feedback loops. In primary hypothyroidism, the thyroid gland fails to produce sufficient hormone, reducing negative feedback and causing TSH concentrations to rise. In central hypothyroidism, the defect lies above the thyroid gland, so appropriate TSH stimulation is absent or biologically inadequate despite low circulating thyroid hormone.
Understanding the anatomical level of dysfunction is essential because similar clinical manifestations can arise from fundamentally different lesions. Evaluation of TSH together with free T4 allows thyroid gland failure to be distinguished from many disorders of the hypothalamic-pituitary system, connecting endocrine anatomy directly with laboratory diagnosis and clinical physiology.
Published on September 30, 2026
Last updated on September 30, 2026
Last updated on September 30, 2026