Iron Part 1 Flashcards
Which proteins are complexed with irons?
- 65% in haemoglobin (Hb)
- 30% ferritin/haemosiderin
- 3.5% myoglobin (Mb)
- 0.5% haem enzymes
- 0.1% transferrin
What is the Physiological Iron Requirement?
- Turnover 20-30 mg/day
-
Absorption 1-2 mg/day
- 5-10% of 10-15 mg in diet
- 1 mg/day menstruation
- 1-2 mg/day pregnancy
- 0.5 mg/day paediatric
-
Losses 1-2 mg/day
- Intestines, Bile, Skin, Urine
What are causes of increased iron absorption?
- Iron deficiency (up to 30%)
- Increased erythropoiesis
- Ingestion of acids, e.g. ascorbate (reduces Fe3+ to Fe2+)
- Pregnancy
What are causes of decreased iron absorption?
- Decreased erythropoiesis
- Ingestion of alkalis
- Ingestion of tea
- Precipitating agents (phytates, phosphates)
- Desferrioxamine
What are features of Intracellular Ferritin?
-
Intracellular storage of bioavailable iron
- Hollow spherical particle found in Hepatocytes, macrophages
- Solid-state core of up to 4000 Fe3+ ions
-
450 kDa, 24 subunits
- 21 kDa ‘H’ chains, predominate in heart/kidney isoforms
- 19 kDa ‘L’ chains, predominate in liver/spleen isoforms
- Ferroxidase activity
-
Denatures to hemosiderin
- Heterogenous aggregate of iron, lysosomal components and other products of intracellular digestion
What are features of plasma ferritin?
- Small amount of ferritin is secreted into plasma
- L-rich, low iron content, glycosylation slows clearance
- No role in iron transport or uptake but correlates with total stores
What does low ferritin indicate?
Low ferritin indicates iron deficiency
What causes High Ferritin?
High ferritin does not necessarily indicate iron overload. Caused by:
- Redistribution
- Increased ferritin synthesis
- Release of tissue ferritin
- Liver disease
- Chronic inflammation
- Malignancy
- Thyrotoxicosis
- Alcohol
- Familial hyperferritinaemia and cataract syndrome
What should be considered with Ferritin >10,000 μg/L?
Consider:
- Still’s disease
- Haemophagocytic Lymphohistiocytosis
What are investigations for a raised serum ferritin?
- Question alcohol intake and other risk factors for liver disease, transfusion history, family history of iron overload and the presence or absence of type 2 diabetes mellitus, obesity and hypertension, as well as for symptoms and signs that may point to an underlying inflammatory or malignant disorder
- Initial investigations: FBC & film, repeat ferritin, TSAT, inflammation markers (e.g. CRP), renal/liver/lipid profiles (abdominal u/s if LFT abnormal), glucose, consider hepatitis serology
- No evidence for therapeutic venesection in non-alcoholic fatty liver disease
What should be done based on the ferritin levels?
- Ferritin <1000 μg/L, normal TSAT, otherwise well: (lifestyle adjustment), repeat in 3–6 months
- Unexplained persistent hyperferritinaemia (especially >1000 μg/L): refer to a hepatologist
What are features of Transferrin?
- 80 kDa, 6% carbohydrate
- Reversibly binds 2x Fe3+ and 2x HCO3-
- 2x N-linked bi/tri-antenarry glycans
- Determines half-life
- Variants – serum/CSF, alcohol, polymorphisms, inborn errors
- Recycled approximately 500 times
How is Serum Iron assessed?
- Predominantly reflects transferrin-bound iron = in-use iron binding capacity
- Requires a fasted sample otherwise if a patient has a large iron load in food, it will go up as a measurement otherwise even if the patient is low in iron
- Colorimetric, atomic absorption spectroscopy, ICP-MS
- (Abdominal x-ray if acute toxic ingestion suspected)
How is Transferrin and Ferritin measured?
Specific immunoassay
What is the Unsaturated Iron Binding Capacity (UIBC)?
- The unused iron binding capacity. Measures the unbound tranferrin
- Measured as the amount of Fe3+ taken up (by transferrin) at alkaline pH
- Fallen out of favour.
What isTotal Iron Binding Capacity (TIBC)?
Total available iron if all binding sites (principally transferrin) fully saturated
Measured as iron concentration when sample saturated with added Fe3+
- If Calculated from UIBC
- TIBC = UIBC + [Iron]
- If Calculated from transferrin
- TIBC (umol/L) = transferrin (g/L) x 24.7
How is transferrin calculated from TIBC?
Transferrin (g/L) =0.0007 x TIBC (ug/L)
How is %transferrin saturation calculated?
- 100 x [Iron]/TIBC
What causes secondary iron overload?
- Excess iron intake
- Dietary: Drinking beer brewed in iron containers
- Supplements or injections
- Transfusion-related siderosis
- Long-term dialysis
- Sideroblastic anaemia
- Ineffective erythropoiesis
- Porphyria cutanea tarda
What causes Primary Iron Overload?
- Hereditary haemochromatosis: HFE, TFR2 & gain-of-function Ferroportin defects\
- African iron overload
- Juvenile haemochromatosis: Haemojuvelin (BMP coreceptor) & Hepcidin defects
- Neonatal haemochromatosis
- Acaeruloplasminaemia
- Hyperferritinaemia-catarract syndrome: Ferritin L-chain defects
- Erythropoietic protoporphyria: Mitoferrin defects
- Atransferrinaemia: Transferrin defects
- Macrophage-predominant iron overload:
- Ferroportin loss-of-function
- Anaemia with iron overload and sideroblasts: Glutaredoxin 5 defects
- Friedreich ataxia: Frataxin defects (mitochondrial iron chaperone)
What is the pathogenesis of Hereditary haemochromatosis?
- Inherited progressive iron overload due to increased iron uptake 3-4 mg/day: positive iron balance 400-1000 mg/year
- Lipid peroxidation in the liver so cellular injury leading to fibrosis
- Iron deposition: Liver (90%), Endocrine glands, Joints, Heart
- Menstruation has positive effect.
What is the classican triad of Hereditary Haemochromatosis?
- Pigmentation (‘bronzing’)
- Diabetes
- Cirrhosis
- Hypothyroidism
- Hypogonadism
- Cardiomyopathy
What are common symptoms of Hereditary haemochromatosis?
- Fatigue
- Weight loss
- Weakness
- Arthralgia
- Oligo/amenorrhoea
- Erectile dysfunction
What are investigations for Hereditary haemochromatosis?
- FBC, LFT, TSAT
- High transferrin saturation and serum ferritin
- (Liver biopsy, hepatic stored iron)
- MRI scan
- (Genetic testing): HFE gene defects have low penetrance. Only C282Y homozygotes generally develop clinical disease
- Exclude other causes of iron overload
What are the genetics of Hereditary Haemachromatosis?
HFE gene defects predominate: Very high prevalence in north European descent (polymorphisms)
-
HFE C282Y: HFE protein cannot bind B2-microglobulin
- HFE cannot stabilise in cell membranes
- Unregulated TFR-mediated intestinal iron uptake
-
HFE H64D
- HFE binds TFR but lacks high degree of inhibition
What the different variations of HFE gene defects?
1 copy C282Y
- Carrier of the C282Y variant; no increased risk of developing hereditary HC
- 25% of individuals may exhibit mild/moderate iron overload
- Complications due to iron overload are rare
- May be influenced by additional factors( genetic, environmental)
1 copy C282Y, 1 copy H63D
- Excludes the most common cause of hereditary HC
- May predispose to mild/moderate iron overload
- If iron overload, consider other contributing factors
- If severe iron overload, consider rare genetic causes
- Consider family testing for iron overload
2 copies C282Y
- Reported in approximately 90% of hereditary HC patients
- Investigate for iron overload
- Consider family testing for iron overload
What is the treatment for Hereditary Haemachromatosis?
Reduce the amount of iron present
- Therapeutic phlebotomy - weekly venesection and FBC
- Ferritin 20-30 μg/L and TSAT <50% (monthly TSAT)
- Blood donation opportunity
- Iron chelation (desferrioxamine)
Avoid alcohol
‘Minihepcidins’
