72,234 articles
Hypothalamic–pituitary–adrenal axis
Hypothalamic–pituitary–adrenal (HPA) axis: neuroendocrine system linking hypothalamus, anterior pituitary, and adrenal cortex. CRH/AVP drive ACTH and cortisol; glucocorticoid negative feedback, circadian and hourly ultradian rhythms, stress response, and suppression by chronic glucocorticoids.
Hypothalamic–pituitary–gonadal axis
Hypothalamic–pituitary–gonadal (HPG) axis, also called the reproductive endocrine axis: GnRH pulses drive LH/FSH, which drive gonadal steroids with feedback; active in fetal life and mini-puberty, quiescent in childhood, reactivated at puberty.
Hypothalamus
Hypothalamus: small ventral brain region below the thalamus, about 2% of brain volume, controlling the pituitary via releasing hormones (TRH, GnRH, CRH, dopamine) and homeostasis, appetite, temperature, and circadian rhythm; mapped in 2025 as HYPOMAP with 452 cell types.
Hypothenar hammer syndrome
Hypothenar hammer syndrome (HHS, ulnar hammer syndrome): a vascular overuse injury where repeated palm trauma compresses the ulnar artery against the hook of the hamate at Guyon's canal, causing thrombosis, aneurysm, corkscrew angiographic appearance, and digital ischemia in dominant hands of male manual workers.
Hypothermia
Hypothermia, also called accidental hypothermia, is an unintentional drop in core body temperature below 35 °C (95 °F). Classified mild, moderate, severe; causes shivering, confusion, arrhythmias; treated by rewarming, ECLS in arrest.
Hypothermia of newborn
Hypothermia of newborn (neonatal hypothermia, ICD-11 KD12): core temperature below 36.5°C in a newborn. Common worldwide, worsens mortality 28% per degree below 36.5°C at admission; prevented by drying, warming, skin-to-skin care, plastic wrap.
Hypothetical types of biochemistry
Hypothetical types of biochemistry, also called alternative biochemistry, propose life not based on carbon in water: silicon-based life, solvents like ammonia, methane, sulfuric acid, or liquid CO2, and oxygen-free CHN chemistry for worlds such as Titan.
Hypothyroidism
Hypothyroidism: deficiency of thyroid hormone, usually primary (Hashimoto thyroiditis or iodine deficiency). Marked by high TSH with low free T4; treated with levothyroxine ~1.6 µg/kg/day. Also called underactive thyroid.
Hypotonia
Hypotonia (decreased muscle tone, floppy infant syndrome) is abnormally low resistance to passive movement, distinct from weakness. Causes are central (60-80% of neonatal cases) or peripheral neuromuscular; evaluation uses clinical signs and targeted testing.
Hypotonia-cystinuria syndrome
Hypotonia-cystinuria syndrome (HCS, OMIM 606407) is a rare autosomal recessive 2p21 contiguous gene deletion of SLC3A1 and PREPL causing neonatal hypotonia, cystinuria type 1, kidney stones, growth hormone deficiency, and hyperphagia in late childhood.
Hypotrichosis simplex
Hypotrichosis simplex, also known as hereditary hypotrichosis simplex, is a rare inherited hair-loss condition causing progressive thinning of scalp and body hair from childhood, with no other abnormalities.
Hypotrichosis simplex of the scalp
Hypotrichosis simplex of the scalp (HSS, OMIM 146520) is a rare autosomal dominant alopecia with normal hair at birth, progressive scalp-only hair loss from childhood, caused by nonsense CDSN mutations.
Hypotrichosis with juvenile macular degeneration
Hypotrichosis with juvenile macular degeneration (HJMD, OMIM 601553), a very rare autosomal recessive CDH3/P-cadherin disorder: sparse short hair from birth, progressive macular degeneration, blindness in the second to fourth decades; about 50 patients reported.
Hypotrichosis-deafness syndrome
Hypotrichosis-deafness syndrome, or hypotrichosis-hearing loss syndrome, is a rare autosomal dominant GJB2 (connexin 26) disorder with erythrokeratoderma, sparse hair, nail dystrophy, and sensorineural deafness.
Hypotrichosis-lymphedema-telangiectasia-renal defect syndrome
Hypotrichosis-lymphedema-telangiectasia-renal defect syndrome (HLTRS, OMIM 137940) is an ultra-rare SOX18-related lymphedema disorder with sparse hair, lower-limb lymphedema, palmar telangiectasia, and childhood renal failure, inherited dominantly or recessively.
Hypoventilation
Hypoventilation (respiratory depression, ventilatory failure): inadequate alveolar ventilation causing hypercapnia, from respiratory muscle weakness, chest wall disorders, obesity hypoventilation syndrome, or suppressed drive; treated with CPAP, NIV, or ventilation.
Hypovolaemic shock
Hypovolemic shock is a life-threatening condition in which loss of blood or plasma, most often from trauma or bleeding, leaves too little circulating volume for adequate tissue perfusion.
Hypovolemia
Hypovolaemia (hypovolemia), ICD-11 5C70.1, is decreased blood plasma volume from hemorrhage, burns, or fluid losses; severe cases cause hypovolemic shock treated with crystalloid resuscitation.
Hypovolemic shock
Hypovolemic shock: circulatory shock from critical intravascular volume loss (hemorrhage, GI, renal, or skin fluid losses), causing low preload and tissue hypoperfusion; treated with crystalloid or blood resuscitation and control of the source.
Hypoxanthine guanine phosphoribosyltransferase partial deficiency
Hypoxanthine guanine phosphoribosyltransferase partial deficiency (Kelley-Seegmiller syndrome, Lesch-Nyhan variant, OMIM 300323) is an X-linked HPRT1 disorder causing uric acid overproduction, gout, and variable neurological signs without self-injury.
Hypoxanthine-guanine phosphoribosyltransferase
Hypoxanthine phosphoribosyl transferase (HPRT, HGprt, EC 2.4.2.8), X-linked HPRT1 enzyme recycling hypoxanthine and guanine to IMP and GMP via PRPP; deficiency causes Lesch-Nyhan disease spectrum.
Hypoxemia
Hypoxemia: low partial pressure of oxygen in arterial blood (PaO2 < 60 mm Hg, SaO2 < 90%), distinct from tissue hypoxia. Five mechanisms: V/Q mismatch, shunt, diffusion limitation, hypoventilation, low inspired PO2.
Hypoxia (medicine)
Hypoxia (medicine): insufficient oxygen in body tissues, distinct from hypoxemia. Four types: hypoxemic, circulatory, anemic, histotoxic (cyanide). Detected by pulse oximetry and arterial blood gas; treated by addressing the cause.
Hypromellose acetate succinate
Hypromellose acetate succinate (HPMCAS, Shin-Etsu AQOAT) is a cellulose-derived enteric pharmaceutical excipient, marketed since 1986, in L/M/H grades dissolving at pH 5.5/6.0/6.5, used for enteric coating and amorphous solid dispersions; GRAS notice GRN 960 closed 2021.
Hypsugo
Hypsugo (Savi's pipistrelle genus) is a genus of small vespertilionid bats in tribe Vespertilionini, type species Hypsugo savii, with roughly 16-18 species across the Palaearctic and Indo-Malayan regions, formerly often merged into Pipistrellus.
Hyptis
Hyptis (Jacq., 1787) is a conserved Lamiaceae genus of 169 accepted species native to tropical and subtropical America, introduced across Africa and Asia; aromatic, terpene-rich, including H. suaveolens and H. capitata, the type species.
Hyracotherium
Hyracotherium, formerly Eohippus, the dawn horse, is an early Eocene small perissodactyl (55–45 Ma) from North America and Europe, 30–60 cm tall, 5–35 kg, four-toed in front, once classed as the first horse but now excluded from the horse lineage.
Hyrax
Hyrax (Hyracoidea, dassies): small rodent-like African and Middle Eastern mammals whose closest relatives are elephants and manatees; rock hyrax Procavia capensis lives in colonies on kopjes, builds middens, Least Concern.
Hyriidae
Hyriidae: a family of about 90 freshwater mussel species (Unionida, Hyrioidea) with a disjunct Gondwanan distribution in Australasia and South America; Cretaceous origin, glochidia larvae parasitic on fish hosts such as Gobiomorphus.
Hyssopus officinalis
Hyssopus officinalis (hyssop), Mediterranean mint-family herb with blue-violet flowers; culinary flavoring and Chartreuse ingredient; pinocamphone-containing essential oil is convulsant; one asthma trial.