Functional Hypothalamic Amenorrhea: This condition is characterized by the cessation of menstrual cycles due to stress, weight loss, or excessive exercise, leading to a disruption in the hypothalamic-pituitary-gonadal axis. Given the patient's age, irregular periods, and the absence of other clear causes, this diagnosis is plausible, especially if the patient has experienced recent stress, significant weight changes, or engages in intense physical activity.

Thyroid Dysfunction (despite normal TFTs initially): Although the patient's thyroid function tests (TFTs) are normal, thyroid issues can sometimes present with fluctuating hormone levels. It's possible that the initial normal TFTs might not reflect the patient's thyroid status accurately, especially if the condition is intermittent or if the patient has recently started or changed thyroid medication.
Pituitary Tumors: Although less common, pituitary tumors (like prolactinomas) can cause amenorrhea by disrupting the normal secretion of gonadotropin-releasing hormone (GnRH), leading to decreased secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The absence of biochemical markers for PCOS and normal weight does not rule out this possibility.
Hyperandrogenism not related to PCOS: Conditions like congenital adrenal hyperplasia or androgen-secreting tumors could cause irregular periods and might not always present with the typical biochemical markers of PCOS.

Pregnancy (despite negative test): Although the patient has had a negative pregnancy test, it's crucial to consider the possibility of a very early pregnancy or a rare condition like a pseudopregnancy. Repeating the pregnancy test after a few days can help rule out this possibility more conclusively.
Hemochromatosis: This genetic disorder leads to excessive iron accumulation in the body and can cause endocrine dysfunction, including amenorrhea. The raised bilirubin could be a sign of liver involvement, but this would be an uncommon presentation.
Wilson's Disease: A genetic disorder that leads to copper accumulation in the body, potentially affecting the liver and brain. It can cause menstrual irregularities among other symptoms and is important to diagnose early due to its potential for severe complications.

Sheehan Syndrome: A rare condition that occurs in postpartum women, characterized by necrosis of the pituitary gland, usually following severe postpartum hemorrhage. Given the patient's age and presentation, this would be highly unusual unless she has had a recent, unreported pregnancy.
Autoimmune Oophoritis: An autoimmune condition where the ovaries are targeted, potentially leading to ovarian failure. This is a rare cause of amenorrhea and would typically be associated with other autoimmune symptoms or conditions.
Gonadal Dysgenesis: A condition where the ovaries do not develop normally, which could lead to irregular periods or amenorrhea. This diagnosis might be considered if there are other signs of gonadal dysgenesis or if karyotyping reveals an abnormality.