The Thyroid Gland is a butterfly-shaped gland located in the anterior neck, below the larynx (Adam's apple), wrapping around the trachea. It has two lobes connected by an isthmus. Weight: ~25-30 g. It is the largest purely endocrine gland in the body. Produces: T3 (Triiodothyronine) = More active form. T4 (Thyroxine) = Main form secreted; converted to T3 in tissues. Calcitonin = Produced by parafollicular C-cells - lowers blood calcium (by inhibiting bone resorption). Requires iodine for T3/T4 synthesis. Regulated by TSH from anterior pituitary (controlled by TRH from hypothalamus).

Glucagon is a peptide hormone produced by alpha cells of the islets of Langerhans. Released when blood glucose falls (hypoglycaemia, fasting, exercise). Actions - all CATABOLIC (breaking down) and hyperglycaemic (glucose-raising): Glycogenolysis (breaks down liver glycogen - releases glucose into blood), Gluconeogenesis (synthesis of new glucose from amino acids, lactate, glycerol), Lipolysis (breaks down fat - releases fatty acids for energy), and Ketogenesis (promotes ketone body formation from fatty acids). Glucagon and Insulin are antagonistic - they maintain blood glucose homeostasis together. Clinical use: IV/IM glucagon to treat severe hypoglycaemia (unconscious diabetic patient) when IV access not available.

Calcitonin is produced by parafollicular C-cells (clear cells) of the thyroid gland (not follicular cells - which produce T3/T4). It is released when blood calcium rises. Actions - all calcium-lowering (opposite of PTH): Inhibits osteoclasts causing reduced bone resorption causing reduced calcium release from bone into blood, Increases renal calcium excretion (minor effect), and Inhibits intestinal calcium absorption (minor effect). In humans, calcitonin plays a relatively minor physiological role (PTH is far more important for calcium regulation). However, calcitonin is clinically important as a tumour marker for Medullary Thyroid Carcinoma (MTC) - which arises from C-cells. Salmon calcitonin is used clinically to treat Paget's disease of bone and hypercalcaemia of malignancy.

Thyroid Hormones (T3 and T4) are essential for: Increased Basal Metabolic Rate (BMR) - stimulate O2 consumption and heat production (calorigenic effect), Growth and development - essential for normal brain development in infants and children (deficiency causes cretinism), Protein synthesis - promote growth of muscles and organs, Heart rate - increase heart rate and cardiac output, Gut motility - stimulate peristalsis, Temperature regulation - increase heat production, and Nervous system - maintain alertness, reflexes, and mood. T3 is 3-5 times more potent than T4 but T4 is more abundant. Both are lipid-soluble and travel bound to TBG (Thyroxine Binding Globulin) in blood.

Type 1 Diabetes Mellitus (T1DM) - previously called Juvenile-onset or Insulin-Dependent Diabetes Mellitus (IDDM). Cause: Autoimmune destruction of pancreatic beta cells by T-lymphocytes causing absolute insulin deficiency (no insulin production). Features: Usually presents in childhood/adolescence (but can occur at any age), Thin patient (loss of fat and muscle due to catabolism), Polyuria (osmotic diuresis), Polydipsia (thirst), Polyphagia (hunger - cells starving), Weight loss, Prone to DKA (Diabetic Ketoacidosis) - life-threatening emergency, Anti-GAD, Anti-islet cell, Anti-insulin antibodies positive, and Genetic association: HLA-DR3, HLA-DR4. Treatment: Lifelong insulin therapy (multiple daily injections or insulin pump).

FSH (Follicle Stimulating Hormone) is a gonadotrophin produced by gonadotroph cells of the anterior pituitary, regulated by GnRH (Gonadotrophin Releasing Hormone) from the hypothalamus. Functions: In Females = Stimulates follicular development in the ovary causing follicles to produce oestrogen and prepares for ovulation. In Males = Stimulates Sertoli cells in the testes causing promotion of spermatogenesis (sperm production). FSH and LH work together - FSH sets the stage, LH triggers the event (ovulation). FSH is elevated in primary gonadal failure (ovarian/testicular failure) because negative feedback from sex hormones is lost. Low FSH indicates secondary (pituitary/hypothalamic) cause of hypogonadism. Clinical use: Fertility treatments (IVF - ovarian stimulation uses FSH injections).

Hypothyroidism is a condition of insufficient thyroid hormone production. Features (mnemonic SLOW): Slow metabolism (weight gain, constipation, bradycardia), Low energy (fatigue, lethargy, depression), Oedema (myxoedema - non-pitting oedema - puffy face, periorbital oedema), and Warm clothing needed (cold intolerance, dry skin, hair loss). Most common cause: Hashimoto's thyroiditis (autoimmune - anti-TPO antibodies) in iodine-sufficient countries. Iodine deficiency = most common worldwide cause causing Goitre. In neonates: Cretinism (intellectual disability, short stature, deafness). Treatment: Levothyroxine (T4 replacement). Diagnosis: TSH elevated + Low T4.

Type 2 Diabetes Mellitus (T2DM) - previously called Adult-onset or Non-Insulin-Dependent Diabetes Mellitus (NIDDM). Most common type (~90-95% of all diabetes). Cause: Insulin resistance (cells don't respond to insulin) + progressive beta cell failure causing relative insulin deficiency. Risk factors: Obesity (especially central/abdominal), physical inactivity, family history, age >45, ethnicity (South Asian, Afro-Caribbean). Features: Often asymptomatic for years (insidious onset), less prone to DKA but can develop HHS (Hyperosmolar Hyperglycaemic State). Complications: Macro (heart disease, stroke, peripheral vascular disease) and Micro (retinopathy, nephropathy, neuropathy). Treatment: Lifestyle changes (diet, exercise, weight loss), Metformin (first-line drug), then additional agents (SGLT2 inhibitors, GLP-1 agonists, etc.).

LH (Luteinising Hormone) is a gonadotrophin from the anterior pituitary (regulated by GnRH). Functions: In Females = Triggers ovulation (LH surge around day 14 of the menstrual cycle causing follicle rupture and egg release). After ovulation, LH stimulates the ruptured follicle to become the corpus luteum, and corpus luteum produces progesterone (maintains uterine lining for potential pregnancy). In Males = Stimulates Leydig cells in the testes to produce testosterone. LH surge is detected by ovulation predictor kits (OPKs) - used to time intercourse for conception. Polycystic Ovary Syndrome (PCOS): Elevated LH:FSH ratio (>2:1), excess androgens, anovulation, and cystic ovaries.

Hyperthyroidism is a condition of excess thyroid hormone production. Features (mnemonic FAST): Fast metabolism (weight loss despite increased appetite, frequent bowel movements), Agitated heart (tachycardia, palpitations, atrial fibrillation), Sweating and heat intolerance, tremor, anxiety, insomnia, and Thyroid enlargement (goitre). Most common cause: Graves' Disease (autoimmune - TSI/TRAb antibodies stimulate TSH receptors causing excess T3/T4). Specific signs of Graves': Exophthalmos (bulging eyes - proptosis), pretibial myxoedema, thyroid acropachy. Treatment: Antithyroid drugs (carbimazole, propylthiouracil), Radioactive iodine (131I), Surgery (thyroidectomy). Beta-blockers (propranolol) for symptom control.