Autoimmune polyendocrine syndrome type II (APS2), or Schmidt syndrome, is characterized by the presence of autoimmune Addison disease in association with either autoimmune thyroid disease or type I diabetes mellitus, or both. Chronic candidiasis is not present. APS2 may occur at any age and in both sexes, but is most common in middle-aged females and is very rare in childhood (summary by Betterle et al., 2004). See 240300 for a phenotypic description of autoimmune polyendocrine syndrome type I (APS1).

Fibrous dysplasia / McCune-Albright syndrome (FD/MAS), the result of an early embryonic postzygotic somatic activating pathogenic variant in GNAS (encoding the cAMP pathway-associated G-protein, Gsa), is characterized by involvement of the skin, skeleton, and certain endocrine organs. However, because Gsa signaling is ubiquitous, additional tissues may be affected. Café au lait skin macules are common and are usually the first manifestation of the disease, apparent at or shortly after birth. Fibrous dysplasia (FD), which can involve any part and combination of the craniofacial, axial, and/or appendicular skeleton, can range from an isolated, asymptomatic monostotic lesion discovered incidentally to severe disabling polyostotic disease involving practically the entire skeleton and leading to progressive scoliosis, facial deformity, and loss of mobility, vision, and/or hearing. Endocrinopathies include: Gonadotropin-independent precocious puberty resulting from recurrent ovarian cysts in girls and autonomous testosterone production in boys; Testicular lesions with or without associated gonadotropin-independent precocious puberty; Thyroid lesions with or without non-autoimmune hyperthyroidism; Growth hormone excess; FGF23-mediated phosphate wasting with or without hypophosphatemia in association with fibrous dysplasia; and Neonatal hypercortisolism. The prognosis for individuals with FD/MAS is based on disease location and severity.

Glutaric aciduria III is characterized by an isolated accumulation of glutaric acid. It appears to be a 'non-disease' as it is found in healthy individuals and is associated with inconsistent symptoms in others (summary by Marlaire et al., 2014).

Andersen-Tawil syndrome (ATS) is characterized by a triad of: episodic flaccid muscle weakness (i.e., periodic paralysis); ventricular arrhythmias and prolonged QT interval; and anomalies including low-set ears, widely spaced eyes, small mandible, fifth-digit clinodactyly, syndactyly, short stature, and scoliosis. Affected individuals present in the first or second decade with either cardiac symptoms (palpitations and/or syncope) or weakness that occurs spontaneously following prolonged rest or following rest after exertion. Mild permanent weakness is common. Mild learning difficulties and a distinct neurocognitive phenotype (i.e., deficits in executive function and abstract reasoning) have been described.

A rare hyperthyroidism characterized by mild to severe hyperthyroidism, presence of goiter, absence of features of autoimmunity, frequent relapses while on treatment and a positive family history.

Selective pituitary resistance to thyroid hormone (PRTH) results in continued thyroid-stimulating hormone (TSH) production driving hypersecretion of T3 and T4 to establish a new equilibrium, with high serum levels of free thyroid hormones together with a nonsuppressed TSH. The presence of a variety of thyrotoxic features, including palpitations, anxiety, tremor, heat intolerance, insomnia, weight loss, and increased stool frequency, suggests that peripheral tissues are less refractory to thyroid hormones than the pituitary (summary by Adams et al., 1994).

Some degree of stimulation of the thyroid gland by chorionic gonadotropin (see 118860) is common during early pregnancy. When serum chorionic gonadotropin concentrations are abnormally high, e.g., in women with molar pregnancies (231090), overt hyperthyroidism may ensue. The pathophysiologic mechanism appears to be promiscuous stimulation of the thyrotropin receptor by the excess chorionic gonadotropin. The explanation for this stimulation is the close structural relations between chorionic gonadotropin and thyrotropin and between their receptors (Grossmann et al., 1997).

Thyrotoxic periodic paralysis is a sporadic muscle disorder characterized by episodic attacks of weakness associated with hypokalemia in individuals with hyperthyroidism. The paralysis resolves upon treatment of hyperthyroidism. The disorder is most common among males of Asian descent, including Chinese, Japanese, Vietnamese, Filipino, and Koreans, although it occurs less commonly in individuals of Caucasian background. Thyrotoxic periodic paralysis is clinically similar to hereditary hypokalemic periodic paralysis (HOKPP; 170400), but the paralysis in TTPP occurs only in the presence of hyperthyroidism. TTPP can also be precipitated by factors that result in hypokalemia, such as carbohydrate ingestion and rest after exercise (review by Kung, 2006). Genetic Heterogeneity of Thyrotoxic Periodic Paralysis See also TTPP2 (613239), conferred by variation in the KCNJ18 gene (613236) on chromosome 17p11, and TTPP3 (614834), mapped to chromosome 17q24.

Any thyrotoxic periodic paralysis in which the cause of the disease is a mutation in the KCNJ18 gene.

PIK3CA-related overgrowth spectrum (PROS) encompasses a range of clinical findings in which the core features are congenital or early-childhood onset of segmental/focal overgrowth with or without cellular dysplasia. Prior to the identification of PIK3CA as the causative gene, PROS was separated into distinct clinical syndromes based on the tissues and/or organs involved (e.g., MCAP [megalencephaly-capillary malformation] syndrome and CLOVES [congenital lipomatous asymmetric overgrowth of the trunk, lymphatic, capillary, venous, and combined-type vascular malformations, epidermal nevi, skeletal and spinal anomalies] syndrome). The predominant areas of overgrowth include the brain, limbs (including fingers and toes), trunk (including abdomen and chest), and face, all usually in an asymmetric distribution. Generalized brain overgrowth may be accompanied by secondary overgrowth of specific brain structures resulting in ventriculomegaly, a markedly thick corpus callosum, and cerebellar tonsillar ectopia with crowding of the posterior fossa. Vascular malformations may include capillary, venous, and less frequently, arterial or mixed (capillary-lymphatic-venous or arteriovenous) malformations. Lymphatic malformations may be in various locations (internal and/or external) and can cause various clinical issues, including swelling, pain, and occasionally localized bleeding secondary to trauma. Lipomatous overgrowth may occur ipsilateral or contralateral to a vascular malformation, if present. The degree of intellectual disability appears to be mostly related to the presence and severity of seizures, cortical dysplasia (e.g., polymicrogyria), and hydrocephalus. Many children have feeding difficulties that are often multifactorial in nature. Endocrine issues affect a small number of individuals and most commonly include hypoglycemia (largely hypoinsulinemic hypoketotic hypoglycemia), hypothyroidism, and growth hormone deficiency.

\n\nThe features of Cowden syndrome overlap with those of another disorder called Bannayan-Riley-Ruvalcaba syndrome. People with Bannayan-Riley-Ruvalcaba syndrome also develop hamartomas and other noncancerous tumors.  Some people with Cowden syndrome have relatives diagnosed with Bannayan-Riley-Ruvalcaba syndrome, and other affected individuals have the characteristic features of both conditions. Based on these similarities, researchers have proposed that Cowden syndrome and Bannayan-Riley-Ruvalcaba syndrome represent a spectrum of overlapping features known as PTEN hamartoma tumor syndrome (named for the genetic cause of the conditions) instead of two distinct conditions.\n\nSome people do not meet the strict criteria for a clinical diagnosis of Cowden syndrome, but they have some of the characteristic features of the condition, particularly the cancers. These individuals are often described as having Cowden-like syndrome. Both Cowden syndrome and Cowden-like syndrome are caused by mutations in the same genes.\n\nCowden syndrome is associated with an increased risk of developing several types of cancer, particularly cancers of the breast, a gland in the lower neck called the thyroid, and the lining of the uterus (the endometrium). Other cancers that have been identified in people with Cowden syndrome include kidney cancer, colorectal cancer, and an agressive form of skin cancer called melanoma. Compared with the general population, people with Cowden syndrome develop these cancers at younger ages, often beginning in their thirties or forties. People with Cowden syndrome are also more likely to develop more than one cancer during their lifetimes compared to the general population. Other diseases of the breast, thyroid, and endometrium are also common in Cowden syndrome. Additional signs and symptoms can include an enlarged head (macrocephaly) and a rare, noncancerous brain tumor called Lhermitte-Duclos disease. A small percentage of affected individuals have delayed development, intellectual disability, or autism spectrum disorder, which can affect communication and social interaction.\n\nAlmost everyone with Cowden syndrome develops hamartomas. These growths are most commonly found on the skin and mucous membranes (such as the lining of the mouth and nose), but they can also occur in the intestine and other parts of the body. The growth of hamartomas on the skin and mucous membranes typically becomes apparent by a person's late twenties.\n\nCowden syndrome is a genetic disorder characterized by multiple noncancerous, tumor-like growths called hamartomas and an increased risk of developing certain cancers.

Mitochondrial myopathy and ataxia (MMYAT) is an autosomal recessive mtDNA depletion disorder characterized by cerebellar ataxia, congenital muscle involvement with histologic findings ranging from myopathic to dystrophic, and pigmentary retinopathy (summary by Donkervoort et al., 2019).