Hypothyroidism is a state in which insufficient thyroid hormone action is available to meet the body's physiological requirements. It most commonly results from primary thyroid-gland failure, but it can also arise from inadequate stimulation by the hypothalamus or pituitary, congenital abnormalities, iodine imbalance, medication effects or treatment that damages thyroid tissue. In primary Hypothyroidism, the thyroid cannot produce adequate hormone, so reduced circulating free T4 removes negative feedback on the pituitary and causes thyroid-stimulating hormone to rise.
In central Hypothyroidism, the pituitary or hypothalamus fails to provide adequate stimulation, so TSH is low, normal or inappropriately modest relative to the low free T4. Hashimoto thyroiditis is a major cause of primary Hypothyroidism in iodine-sufficient populations and involves autoimmune destruction or functional impairment of thyroid tissue. Hypothyroidism reduces metabolic activity and affects multiple organ systems, producing manifestations such as fatigue, cold intolerance, constipation, dry skin, slowed heart rate, cognitive slowing and changes in lipid metabolism. The severity and symptom profile vary considerably between individuals.
Thyroid hormones influence transcription of genes involved in mitochondrial function, energy expenditure, carbohydrate and lipid metabolism, cardiovascular physiology and nervous-system function. In Hypothyroidism, reduced thyroid-hormone signalling lowers basal metabolic activity and changes the function of tissues throughout the body. Cardiovascular effects can include reduced heart rate and cardiac output and increased systemic vascular resistance, while lipid metabolism may be altered, contributing to raised low-density lipoprotein cholesterol in many patients. Gastrointestinal motility may decrease, producing constipation.
Skin can become dry because of altered epidermal and dermal processes, and hair may become coarse or thin. Neuromuscular effects can include slowed reflex relaxation, muscle weakness, cramps or aches. Accumulation of hydrophilic substances in connective tissues can contribute to the characteristic non-pitting oedema historically associated with severe Hypothyroidism. These effects arise from reduced hormone action rather than from a single organ-specific defect.
Hashimoto thyroiditis is an autoimmune cause of Hypothyroidism in which immune responses target thyroid components, commonly including thyroid peroxidase and thyroglobulin. Over time, chronic inflammation and immune-mediated injury can reduce functional thyroid tissue. Some patients initially have normal thyroid hormone concentrations with elevated TSH, a state known as subclinical hypothyroidism, whereas others develop overt Hypothyroidism with low free T4. The disease can coexist with other autoimmune conditions, reflecting shared susceptibility rather than a single isolated mechanism.
Postpartum thyroid dysfunction can also involve an autoimmune inflammatory process and may produce a transient hyperthyroid phase followed by Hypothyroidism before recovery in some individuals. Iatrogenic Hypothyroidism can occur after radioactive iodine treatment, thyroid surgery or external radiation involving the gland. Certain medicines can interfere with hormone synthesis or metabolism.
Diagnosis generally relies on thyroid-function tests interpreted in context. In primary Hypothyroidism, TSH is typically elevated and free T4 is reduced when the condition is overt. In subclinical Hypothyroidism, TSH is elevated while free T4 remains within the laboratory reference range. The distinction is important because the potential benefits and indications for treatment differ according to TSH concentration, symptoms, age, pregnancy status, cardiovascular considerations and other factors.
Thyroid peroxidase antibodies can support an autoimmune diagnosis and may help estimate the likelihood of progression from subclinical to overt disease, although antibody positivity does not by itself establish that treatment is required. Central Hypothyroidism requires particular attention because TSH may not be appropriately elevated despite low free T4. Clinical interpretation should also account for severe non-thyroidal illness, medications and assay interference, all of which can alter thyroid-test results without representing straightforward primary thyroid failure.
Levothyroxine is the standard replacement therapy for most forms of persistent primary Hypothyroidism. It is synthetic T4 and is converted by tissues into T3, allowing the body to regulate local thyroid-hormone action through normal physiological pathways. Dose requirements depend on factors including age, body size, residual thyroid function, pregnancy, interacting medicines and underlying cause. Absorption can be reduced by certain foods, supplements and gastrointestinal conditions, and medicines containing iron or calcium can interfere when taken too close to levothyroxine.
Consistent administration and appropriate monitoring are therefore important. TSH is commonly used to monitor treatment in primary Hypothyroidism because it reflects pituitary feedback to circulating thyroid hormone. Treatment targets and monitoring intervals vary according to the clinical setting, and pregnancy requires particular attention because maternal thyroid hormone availability is important for fetal development.
Untreated or inadequately treated Hypothyroidism can produce progressively broader physiological consequences. Severe longstanding deficiency can cause marked somnolence, hypothermia, bradycardia, hypoventilation and altered consciousness in the rare emergency state known as myxoedema coma. Cardiovascular risk can be affected through changes in lipid metabolism and vascular physiology. Reproductive consequences can include menstrual disturbances and impaired fertility, while pregnancy-associated Hypothyroidism can have implications for maternal and fetal health.
In children, thyroid hormone deficiency can impair growth and neurodevelopment, which is why newborn screening programmes identify congenital Hypothyroidism before severe developmental consequences occur. Congenital disease may result from thyroid dysgenesis, abnormal hormone synthesis or other developmental mechanisms. Early treatment in infancy is highly effective at preventing the severe developmental consequences of prolonged hormone deficiency.
The clinical presentation of Hypothyroidism is often gradual, and symptoms may be subtle or attributed to ageing, stress or other conditions. Fatigue, weight gain, constipation, dry skin, cold intolerance and low mood are common associations but are not specific enough to establish the diagnosis. Some patients have relatively few symptoms despite clearly abnormal biochemical tests, while others experience substantial symptoms with only modest biochemical abnormalities.
Weight change in Hypothyroidism is often smaller than popularly assumed and may partly reflect fluid retention rather than large increases in adipose tissue. Another misconception is that a normal TSH always excludes Hypothyroidism; this is generally true for primary disease in appropriate circumstances but not necessarily for central Hypothyroidism, where pituitary regulation is impaired. Interpretation therefore depends on understanding the level at which the endocrine axis is disrupted.
Hypothyroidism is closely related to but distinct from euthyroid states, subclinical disease and non-thyroidal illness. A person can have thyroid antibodies without currently having biochemical Hypothyroidism, and a mildly elevated TSH does not necessarily indicate permanent gland failure. Temporary changes can occur during thyroiditis, pregnancy, severe illness or medication exposure. Conversely, a patient taking levothyroxine can have persistent symptoms for reasons unrelated to inadequate thyroid hormone replacement, so symptoms alone should not dictate dose escalation.
Excessive replacement produces iatrogenic hyperthyroidism and can increase risks such as atrial fibrillation and bone loss in susceptible individuals. The central principle is physiological replacement of deficient hormone while avoiding overtreatment. Hypothyroidism therefore illustrates how endocrine diagnosis requires integration of hormone concentrations, feedback relationships, autoimmune status, clinical context and treatment effects rather than relying on isolated symptoms or a single numerical measurement.
