Symptoms of congenital Type III Galactosemia are apparent from birth, but vary in severity depending on whether the peripheral or generalized disease form is present. Symptoms may include:

- Infantile jaundice

- Infantile hypotonia

- Dysmorphic features

- Sensorineural hearing loss

- Impaired growth

- Cognitive deficiencies

- Depletion of cerebellar Purkinje cells

- Ovarian failure (POI) and hypertrophic hypergonadism

- Liver failure

- Renal failure

- Splenomegaly

- Cataracts

Studies of Type III galactosemia symptoms are mostly descriptive, and precise pathogenic mechanisms remain unknown. This is largely due to a lack of functional animal models of classic galactosemia. The recent development of a "Drosophila melanogaster" GALE mutant exhibiting galactosemic symptoms may yield a promising future animal model.

In undiagnosed and untreated children, the accumulation of precursor metabolites due to the deficient activity of galactose 1-phosphate uridylyltransferase (GALT) can lead to feeding problems, failure to thrive, liver damage, bleeding, and infections. The first presenting symptom in an infant is often prolonged jaundice. Without intervention in the form of galactose restriction, infants can develop hyperammonemia and sepsis, possibly leading to shock. The accumulation of galactitol and subsequent osmotic swelling can lead to cataracts which are similar to those seen in galactokinase deficiency. Long-term consequences of continued galactose intake can include developmental delay, developmental verbal dyspraxia, and motor abnormalities. Galactosemic females frequently suffer from ovarian failure, regardless of treatment in the form of galactose restriction.

Often symptoms will arise that indicate the body is not absorbing or making the lipoproteins that it needs. These symptoms usually appear "en masse", meaning that they happen all together, all the time. These symptoms come as follows:

- Failure to thrive/Failure to grow in infancy

- Steatorrhea/Fatty, pale stools

- Frothy stools

- Foul smelling stools

- Protruding abdomen

- Intellectual disability/developmental delay

- Developmental coordination disorder, evident by age ten

- Muscle weakness

- Slurred speech

- Scoliosis (curvature of the spine)

- Progressive decreased vision

- Balance and coordination problems

A variety of neurological symptoms have been associated with carnosinemia. They include: hypotonia, developmental delay, mental retardation, degeneration of axons, sensory neuropathy, tremors, demyelinization, gray matter anomalies, myoclonic seizures, and loss of purkinje fibers.

This disorder causes neurological problems, including mental retardation, brain atrophy and ventricular dilation, myoclonus, hypotonia, and epilepsy.

It is also associated with growth retardation, megaloblastic anemia, pectus excavatum, scoliosis, vomiting, diarrhea, and hepatosplenomegaly.

Galactose epimerase deficiency, also known as GALE deficiency, Galactosemia III and UDP-galactose-4-epimerase deficiency, is a rare, autosomal recessive form of galactosemia associated with a deficiency of the enzyme "galactose epimerase".

Presenting in infancy, symptoms include lack of appetite, vomiting, dehydration, hypotonia and failure to thrive.

This defect leads to a multi-systemic disorder of the connective tissue, muscles, central nervous system (CNS), and cardiovascular system. Homocystinuria represents a group of hereditary metabolic disorders characterized by an accumulation of the amino acid homocysteine in the serum and an increased excretion of homocysteine in the urine. Infants appear to be normal and early symptoms, if any are present, are vague.

Signs and symptoms of homocystinuria that may be seen include the following:

Signs and symptoms of this disorder include low levels of ketones (products of fat breakdown that are used for energy) and low blood sugar (hypoglycemia). Together these signs are called hypoketotic hypoglycemia. People with this disorder typically also have an enlarged liver (hepatomegaly), muscle weakness, and elevated levels of carnitine in the blood.

Short-chain acyl-coenzyme A dehydrogenase deficiency affected infants will have vomiting, low blood sugar, a lack of energy (lethargy), poor feeding, and failure to gain weight and grow. Additional features of this disorder may include poor muscle tone (hypotonia), seizures, developmental delays, and microcephaly. The symptoms of short-chain acyl-CoA dehydrogenase deficiency may be triggered during illnesses such as viral infections. In some cases, signs and symptoms may not appear until adulthood, when some individuals may develop muscle weakness, while other individuals mild symptoms may never be diagnosed.

Tetrahydrobiopterin deficiency (THBD, BHD), also called THB or BH deficiency, is a rare metabolic disorder that increases the blood levels of phenylalanine. Phenylalanine is an amino acid obtained through the diet. It is found in all proteins and in some artificial sweeteners. If tetrahydrobiopterin deficiency is not treated, excess phenylalanine can build up to harmful levels in the body, causing intellectual disability and other serious health problems.

High levels of phenylalanine are present from infancy in people with untreated tetrahydrobiopterin (THB, BH) deficiency. The resulting signs and symptoms range from mild to severe. Mild complications may include temporary low muscle tone. Severe complications include intellectual disability, movement disorders, difficulty swallowing, seizures, behavioral problems, progressive problems with development, and an inability to control body temperature.

It was first characterized in 1975.

The signs of carnitine-acylcarnitine translocase deficiency usually begin within the first few hours of life. Seizures, an irregular heartbeat, and breathing problems are often the first signs of this disorder. This disorder may also cause extremely low levels of ketones (products of fat breakdown that are used for energy) and low blood sugar (hypoglycemia). Together, these two signs are called hypoketotic hypoglycemia. Other signs that are often present include ammonia in the blood (hyperammonemia), an enlarged liver (hepatomegaly), heart abnormalities (cardiomyopathy), and muscle weakness. This disorder can cause sudden infant death.

Prolidase deficiency generally becomes evident during infancy, but initial symptoms can first manifest anytime from birth to young adulthood. The condition results in a very diverse set symptoms, the severity of which can vary significantly between patients, depending on the degree to which prolidase activity is hampered by the individual underlying mutation(s) in each case. It is even possible, though rare, for affected individuals to be asymptomatic, in which case the disorder can only be identified through laboratory screening of the prospective patient and/or their extended family.

One of the signature features of PD is the elimination of high quantities of peptides through urine.

In addition, most of those affected exhibit persistent skin lesions (starting from a mild rash) or ulcers, primarily on the legs and feet, the formation of which normally begins during childhood. Clinically, these, among other dermatological issues, represent the most distinguishing and most frequent symptoms. These may never recede, potentially leading to severe infections that can, in the worst case, necessitate amputation.

PD patients exhibit a weak immune system and markedly elevated vulnerability to infections in general, and particularly those of the respiratory system, leading some who suffer from PD to acquire recurrent lung disease. They may also have an enlarged spleen (splenomegaly), and on some occasions the spleen and liver may both be enlarged (hepatosplenomegaly). Photosensitivity and hyperkeratosis have been associated with PD. Abnormal facial characteristics, consisting of pronounced eyes which are spaced far apart (hypertelorism), a high forehead, a compressed bridge of the nose or saddle nose, and a small lower jaw and chin (micrognathia), are also observed in the majority of cases.

Those affected by PD can also suffer intellectual disabilities (approx. 75% of recorded cases do) ranging from mild to severe – mental development during childhood may therefore progress more slowly.

Galactokinase deficiency, also known as Galactosemia type 2 or GALK deficiency, is an autosomal recessive metabolic disorder marked by an accumulation of galactose and galactitol secondary to the decreased conversion of galactose to galactose-1-phosphate by galactokinase. The disorder is caused by mutations in the GALK1 gene, located on chromosome 17q24. Galactokinase catalyzes the first step of galactose phosphorylation in the Leloir pathway of intermediate metabolism. Galactokinase deficiency is one of the three inborn errors of metabolism that lead to hypergalactosemia. The disorder is inherited as an autosomal recessive trait. Unlike classic galactosemia, which is caused by deficiency of galactose-1-phosphate uridyltransferase, galactokinase deficiency does not present with severe manifestations in early infancy. Its major clinical symptom is the development of cataracts during the first weeks or months of life, as a result of the accumulation, in the lens, of galactitol, a product of an alternative route of galactose utilization. The development of early cataracts in homozygous affected infants is fully preventable through early diagnosis and treatment with a galactose-restricted diet. Some studies have suggested that, depending on milk consumption later in life, heterozygous carriers of galactokinase deficiency may be prone to presenile cataracts at 20–50 years of age.

Hartnup disease manifests during infancy with variable clinical presentation: failure to thrive, photosensitivity, intermittent ataxia, nystagmus, and tremor.

Nicotinamide is necessary for neutral amino acid transporter production in the proximal renal tubules found in the kidney, and intestinal mucosal cells found in the small intestine. Therefore, a symptom stemming from this disorder results in increased amounts of amino acids in the urine.

Pellagra, a similar condition, is also caused by low nicotinamide; this disorder results in dermatitis, diarrhea, and dementia.

Hartnup disease is a disorder of amino acid transport in the intestine and kidneys; otherwise, the intestine and kidneys function normally, and the effects of the disease occur mainly in the brain and skin. Symptoms may begin in infancy or early childhood, but sometimes they begin as late as early adulthood. Symptoms may be triggered by sunlight, fever, drugs, or emotional or physical stress. A period of poor nutrition nearly always precedes an attack. The attacks usually become progressively less frequent with age. Most symptoms occur sporadically and are caused by a deficiency of niacinamide. A rash develops on parts of the body exposed to the sun. Mental retardation, short stature, headaches, unsteady gait, and collapsing or fainting are common. Psychiatric problems (such as anxiety, rapid mood changes, delusions, and hallucinations) may also result.

Abetalipoproteinemia affects the absorption of dietary fats, cholesterol, and certain vitamins. People affected by this disorder are not able to make certain lipoproteins, which are molecules that consist of proteins combined with cholesterol and particular fats called triglycerides. This leads to a multiple vitamin deficiency, affecting the fat-soluble vitamin A, vitamin D, vitamin E, and vitamin K. However, many of the observed effects are due to vitamin E deficiency in particular.

The signs and symptoms of abetalipoproteinemia appear in the first few months of life (because pancreatic lipase is not active in this period). They can include failure to gain weight and grow at the expected rate (failure to thrive); diarrhea; abnormal spiny red blood cells (acanthocytosis); and fatty, foul-smelling stools (steatorrhea). The stool may contain large chunks of fat and/or blood. Other features of this disorder may develop later in childhood and often impair the function of the nervous system. They can include poor muscle coordination, difficulty with balance and movement (ataxia), and progressive degeneration of the retina (the light-sensitive layer in the posterior eye) that can progress to near-blindness (due to deficiency of vitamin A, retinol). Adults in their thirties or forties may have increasing difficulty with balance and walking. Many of the signs and symptoms of abetalipoproteinemia result from a severe vitamin deficiency, especially vitamin E deficiency, which typically results in eye problems with degeneration of the spinocerebellar and dorsal column tracts.

Galactose-1-phosphate uridylyltransferase deficiency, also called galactosemia type 1, classic galactosemia or GALT deficiency, is the most common type of galactosemia, an inborn error of galactose metabolism, caused by a deficiency of the enzyme galactose-1-phosphate uridylyltransferase. It is an autosomal recessive metabolic disorder that can cause liver disease and death if untreated. Treatment of galactosemia is most successful if initiated early and includes dietary restriction of lactose intake. Because early intervention is key, galactosemia is included in newborn screening programs in many areas. On initial screening, which often involves measuring the concentration of galactose in blood, classic galactosemia may be indistinguishable from other inborn errors of galactose metabolism, including galactokinase deficiency and galactose epimerase deficiency. Further analysis of metabolites and enzyme activities are needed to identify the specific metabolic error.

Glycogen storage disease type III presents during infancy with hypoglycemia and failure to thrive. Clinical examination usually reveals hepatomegaly. Muscular disease, including hypotonia and cardiomyopathy, usually occurs later. The liver pathology typically regresses as the individual enter adolescence, as does splenomegaly, should the individual so develop it.

Fumarase deficiency causes encephalopathy, severe mental retardation, unusual facial features, brain malformation, and epileptic seizures due to an abnormally low amount of fumarase in cells. It can initially present with polyhydramnios on prenatal ultrasound. Affected neonates may demonstrate nonspecific signs of poor feeding and hypotonia. Laboratory findings in neonates may indicate polycythemia, leukopenia, or neutropenia. As they age, neurological deficits begin to manifest with seizures, dystonias, and severe developmental delay.

Signs and symptoms of a biotinidase deficiency can appear several days after birth. These include seizures, hypotonia and muscle/limb weakness, ataxia, paresis, hearing loss, optic atrophy, skin rashes (including seborrheic dermatitis and psoriasis), and alopecia. If left untreated, the disorder can rapidly lead to coma and death.

Biotinidase deficiency can also appear later in life. This is referred to as "late-onset" biotinidase deficiency. The symptoms are similar, but perhaps more mild, because if an individual survives the neonatal period they likely have some residual activity of biotin-related enzymes. Studies have noted individuals who were asymptomatic until adolescence or early adulthood. One study pointed out that untreated individuals may not show symptoms until age 21. Furthermore, in rare cases, even individuals with profound deficiencies of biotinidase can be asymptomatic.

Symptom severity is predictably correlated with the severity of the enzyme defect. Profound biotinidase deficiency refers to situations where enzyme activity is 10% or less. Individuals with partial biotinidase deficiency may have enzyme activity of 10-30%.

Functionally, there is no significant difference between dietary biotin deficiency and genetic loss of biotin-related enzyme activity. In both cases, supplementation with biotin can often restore normal metabolic function and proper catabolism of leucine and isoleucine.

The symptoms of biotinidase deficiency (and dietary deficiency of biotin) can be quite severe. A 2004 case study from Metametrix detailed the effects of biotin deficiency, including aggression, cognitive delay, and reduced immune function.

The signs and symptoms of beta-ketothiolase deficiency include vomiting, dehydration, trouble breathing, extreme tiredness, and occasionally convulsions. These episodes are called ketoacidotic attacks and can sometimes lead to coma. Attacks occur when compounds called organic acids (which are formed as products of amino acid and fat breakdown) build up to toxic levels in the blood. These attacks are often triggered by an infection, fasting (not eating), or in some cases, other types of stress.

Babies with this disorder are usually healthy at birth. The signs and symptoms may not appear until later in infancy or childhood and can include poor feeding and growth (failure to thrive), a weakened and enlarged heart (dilated cardiomyopathy), seizures, and low numbers of red blood cells (anemia). Another feature of this disorder may be very low blood levels of carnitine (a natural substance that helps convert certain foods into energy).

Isobutyryl-CoA dehydrogenase deficiency may be worsened by long periods without food (fasting) or infections that increase the body's demand for energy. Some individuals with gene mutations that can cause isobutyryl-CoA dehydrogenase deficiency may never experience any signs and symptoms of the disorder.

In addition to the symptoms associated with immunodeficiency, such as depletion of T-cells, decline of lymphocyte activity, and an abrupt proliferation of both benign and opportunistic infections — PNP-deficiency is often characterized by the development of autoimmune disorders. lupus erythematosus, autoimmune hemolytic anemia, and idiopathic thrombocytopenic purpura have been reported with PNP-deficiency.

Neurological symptoms, such as developmental decline, hypotonia, and mental retardation have also been reported.

Carnosinemia, also called carnosinase deficiency or aminoacyl-histidine dipeptidase deficiency, is a rare autosomal recessive metabolic disorder caused by a deficiency of "carnosinase", a dipeptidase (a type of enzyme that splits dipeptides into their two amino acid constituents).

Carnosine is a dipeptide composed of beta-alanine and histidine, and is found in skeletal muscle and cells of the nervous system. This disorder results in an excess of carnosine in the urine, cerebrospinal fluid (CSF), blood and nervous tissue. Neurological disorders associated with a deficiency of carnosinase, and the resulting carnosinemia ("carnosine in the blood") are common.

Patients generally have a benign course, and typically present with hepatomegaly and growth retardation early in childhood. Mild hypoglycemia, hyperlipidemia, and hyperketosis may occur. Lactic acid and uric acid levels may be normal. However, lactic acidosis may occur during fasting.