Oestradiol
Oestradiol (US: Estradiol) is one of 3 forms of oestrogen. It is the most prevalent form of oestrogen and is found throughout life. Oestradiol is a steroid hormone and is found in males and females. It differs from estrone, estriol, and estetrol which are mostly only present during pregnancy (Alonso & Rosenfield, 2002). Because oestradiol is present almost always throughout a pubescent individual’s life and not just during pregnancy, it is a good biomarker and indicator of oestrogen levels. Detection of too high/low oestradiol may indicate an increased likelihood to diseases of which oestrogens play a role, or diseases that cause reductions in levels of oestrogen.
Oestradiol is mainly synthesised in the ovaries, placenta, and corpus luteum from cholesterol in premenopausal women (Cui et al., 2013). Oestradiol can be synthesised in the liver, heart, skin, and brain. Cholesterol is converted into pregnenolone, then progesterone which is converted to androgens such as testosterone. Testosterone is converted to oestradiol by aromatase (Cui et al., 2013). The structure of oestradiol is shown in figure 1.
Reference ranges vary depending on the detection method used. In the UK, the NHS use radioimmunoassays (RIA) to detect blood serum oestradiol (Leeds Teaching Hospitals, 2017).
Table 1: Reference ranges for oestradiol provided by Leeds Pathology Services as part of Leeds Teaching Hospitals (2017).
Reference ranges for saliva oestradiol are hard to come by. Gandara in 2007 found saliva oestradiol concentrations of a premenopausal woman was between 0-36pmol/L, significantly lower than blood serum concentrations. Other ranges for saliva include 4.77pmol/L to 12.11pmol/L premenopause and 2.38pmol/L to 6.24 in postmenopausal women (Gottfried, n.d.).
Oestradiol is mainly measured in the blood serum. Other research has explored the possibility of testing oestradiol in saliva to some success. Saliva oestradiol has been show to correlate well with free, unbound oestradiol in the plasma (Fiers et al., 2017). Benefits of testing saliva include that sample-taking is not invasive and saliva samples can be taken outside of the clinical setting i.e. at home. Other recent methods to determine un-bound oestradiol concentration in blood serum include equilibrium dialysis-liquid chromatography-tandem mass spectrometry, which has been shown to have good sensitivity and specificity although complex (Ray et al., 2012).
Radioimmunoassays measure oestradiol levels by coating a plate in oestradiol-specific antibodies. Radiolabeled oestradiol is added to the plate and so the antibodies become fully saturated, the plate is washed to remove excess radiolabelled oestradiol and the radioactivity is measured. The patient’s serum is added and their oestradiol causes competition with the radiolabelled oestradiol. The plate is washed to remove unbound components and the radioactivity measured. The change in radioactivity is proportional to the amount of oestradiol in the patient’s sample (Grange et al., 2014). Radioimmunoassays are fast being replaced by ELISAs that have greater sensitivity and specificity due to using secondary enzyme-linked-antibodies in sandwich ELISAs, and do not have the negative of dealing with radioactive material as RIAs do.
Low levels of oestradiol can be an indicator of hypogonadism. Hypogonadism is a reduction in the function of the gonads and can occur in both men and women. In males, Hypogonadism can be a result of the body not producing sufficient levels of testosterone, an androgen. This can be due to the hypothalamic-pituitary-gonadal axis being defective (Ross & Bhasin, 2016). The causes of primary hypogonadism include injury, inflammation, or infection of the testes or ovaries leading to reduced androgen and thus oestrogen production. Defects in the hypothalamus or pituitary glands can be a cause of secondary hypogonadism and are normally due to defective genes (Ross and Bhasin, 2016). Klinefelter syndrome, in which a male has an extra X chromosome (47 XXY) can be a cause of hypogonadism (Høst et al., 2014). Hypogonadism can cause a lack of testosterone in males which is a precursor for oestradiol; helped by the aromatase enzyme (Ventura et al., 2011). Therefore, if there is a lack of testosterone, there will be a lack of oestradiol leading to lower than normal concentrations in blood samples. Low oestradiol levels have been proven to be found in congenital hypogonadism, thus indicating it is a suitable biomarker (Trabado et al., 2011). Tests are vital in diagnosing hypothyroidism as symptoms include regression of secondary sex characteristics, oligospermia (low sperm count), and erectile dysfunction (Seftel, 2006). In addition to the non-gender linked causes aforementioned, in females, causes of hypogonadism include Turner syndrome (45X) where a female is missing part of, or a complete X chromosome (Gawlik et al., 2016). Hypogonadism in females causes a reduction in oestradiol production as the ovaries are the main site of oestrogen production in females. If the ovaries are lacking function, then oestradiol production will be reduced which can be detectable and an indicator of hypogonadism.
Oestradiol is a cardioprotectant. When oestrogens bind to oestrogen nuclear receptors it is thought that the receptor is able to activate the PI3K signalling pathway that has cardioprotective effects (Lagranha et al., 2010). Oestradiol has also been shown to have effects on cardiovascular function such as cardiac hypertrophy which can lead to heart failure (Eickels, van et al., 2001). Oestrogens are involved in regulating the activity of the MAPK pathway which phosphorylates kinases in a signalling cascade in response to stress, causing hypertrophy of the ventricles (Sugden & Clerk 1998). As oestrogens regulate this, if levels are below those in table 1, cardiac hypertrophy poses a higher risk. Oestradiol has also been shown to have inhibitory effects on the renin-angiotensin-aldosterone system. This has been shown to cause a reduction in cardiac hypertrophy (Nickenig et al., 1998).
Oestradiol can also be used as an indicator for menopause in women. Menopause is triggered as a result of the reduction in oestrogen production. As oestrogen is not being produced, FSH and LH are not released and so eggs are not able to mature or be released; ovulation does not occur (O’Hagan et al., 2012). Menopause has numerous symptoms such as hot flushes, headaches, anxiety, and increased risk of metabolic syndromes amongst others (Coyoy et al., 2016). If these symptoms can be proved to be due to menopause, then treatment can begin to alleviate the symptoms. Blood tests can be carried out to identify low levels of oestradiol, and compared to the reference ranges of menopausal women that are also lower than pre-menopausal women. As menopause is caused by reduced oestrogen production, the symptoms can be treated with hormone therapy, more specifically oestrogen can be administered.
The National Toxicology Programme has declared oestrogen a carcinogen (Miller, 2003). Oestrogens promote proliferation of normal and neoplastic epithelium in breasts. It is thought this is done by oestrogen either stimulating cell proliferation through their normal oestrogen-receptor activity; increasing mutation rates through CP450 mediated activation; or by inducing aneuploidy (Russo, J. & and Russo, I. H., 2006). Oestradiol has been shown to cause cancer cells to continue to divide in culture, where as if oestradiol is absent, the cell lines eventually die (Darbre et al., 1983). It is thought that most tumours are dependent on oestrogen for growth. Although oestrogen production in menopausal women reduces, those with breast tumours have high intratumoural concentrations of oestrogen (Licznerska et al., 2008). It is also thought that metabolites of oestrogen and generated free-radicals damage DNA. Depurination has also been shown to be a result of oestrogen metabolites (Miller, 2003). Because of its carcinogenic properties, increased amounts of oestrogen, and thus oestradiol, above reference ranges in the blood can be used to identify if a patient has a higher-risk of getting cancer. The test only identifies possible increased risk and is not definitive as to whether someone has or will have cancer. Oestradiol levels can become increased in patients with gonadotropin-producing neoplasms, however the sensitivity of this tumour marker is disputed (Kirschner et al., 1981).
Deficiencies in oestrogen can be a contributing factor to forming osteoporosis in post menopausal women and elderly men (Riggs, 2000). Bone resorption is caused by an increased number of osteoclast cells and their activity of breaking down bone tissue and releasing the minerals. Osteoclasts have oestrogen receptors that allow oestrogen to regulate osteoclasts and ultimately inhibit bone resorption. Oestrogen also inhibits the release of osteoclast stimulatory factors and promotes inhibitory factors; again inhibiting bone resorption (Krassas & Papadopoulou, 2001). Oestrogen inhibits osteoclast stimulators such as IL-1 IL-6, M-CSF, PGE2, GM-CSF, and TNF-All these stimulators play a role in osteoblasts forming active osteoclasts and thus bone resorption. Oestrogen promotes osteoclast inhibitors such as TGF-that causes osteoclast apoptosis (Riggs, 2000). Low oestradiol levels in post-menopausal women and elderly men can therefore be an indicator of low oestrogen levels and can be used to identify those with a high risk of developing osteoporosis.
As with most of the conditions aforementioned, oestradiol is used as a biomarker to identify individuals with a higher risk of going on to develop a disease. Abnormal levels of oestradiol in serum, and thus oestrogen levels, is not indicative of a specific disease, there can be many causes of increased or decreased oestradiol levels and thus other symptoms must be present for a concrete diagnosis of a disease. The oestradiol biomarker is good for identifying high risk individuals so that preventative measures such as hormone therapy can begin before signs and symptoms to emerge.