Introduction 
Functional hypothalamic amenorrhoea (FHA) is a common cause of amenorrhoea and anovulation1,; it is responsible for up to 3% of primary 2 and 33.5% of secondary amenorrhoea cases.1 FHA is a form of hypogonadotropic hypogonadism caused by hypothalamic-pituitary-ovarian (HPO) axis dysregulation. It is characterised by estradiol deficiency due to reduced hypothalamic gonadotrophin-releasing hormone (GnRH) secretion. Common causes include stress, weight loss and excessive exercise.1
Early FHA diagnosis is crucial to prevent complications such as osteoporosis and infertility secondary to anovulation. FHA features, such as low body mass index (BMI) and psychological stress, can cause complications, if pregnancy does occur; mothers with low BMI are more likely to have miscarriages 3, give birth prematurely and have low birth weight babies.4 Furthermore, prolonged hypoestrogenemia reduces bone mineral density (BMD), causing osteopenia and osteoporosis and has significant negative impacts on the cardiovascular system.
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Eighty three percent of FHA cases are reversible, when causative factors are addressed.
8 The 2017 Endocrine Society Guidelines recommend first-line treatment of FHA should aim to energy availability by improving nutrition, supporting weight gain and reducing exercise.
9 However, FHA diagnosis may be complicated by an indistinct clinical picture and confusion with another common cause of menstrual disturbance: polycystic ovary syndrome (PCOS). The typical hormone profile in PCOS has some reported distinctions from FHA, namely: elevated LH: follicle-stimulating hormone (FSH) ratio and free testosterone; reduced sex hormone binding globulin; and normal estradiol, prolactin, cortisol and thyroid hormones.10 However, many women with FHA or PCOS display non-typical hormone profiles. Furthermore, up to 40% of women with FHA have features of polycystic ovarian morphology (PCOM) on ultrasound.
11,12 PCOS usually presents with oligomenorrhea or eumenorrhea but can also present with amenorrhoea. In contrast to FHA, PCOS treatment involves controlling hyperandrogenism symptoms and reducing metabolic risk. The Endocrine Society recommends hormonal contraceptives in PCOS to regulate periods13, however the oral contraceptive pill is not recommended for FHA since they provide inferior bone mineralisation, when compared with hormone replacement therapy.
9 Failure to detect FHA can also result in failure to address underlying psychological comorbidities.  Few studies have compared the clinical and biochemical features of FHA with other causes of amenorrhoea and there remains limited data on the diagnostic performance of parameters such as estradiol, LH and BMI, in distinguishing between FHA and PCOS.
Our study aimed to investigate the clinical and biochemical features of FHA and comparing it to that of women with PCOS. Additionally, we explored the diagnostic performance of these parameters in distinguishing between FHA and PCOS.