| Fetal/Congenital | Thyroid hormone essential for brain myelination and skeletal development; maternal iodine → fetal T4 | — | Cretinism (congenital hypothyroidism): stunted growth, irreversible mental retardation |
| Infancy (0–2 yr) | Thymus maximally active; T-cell education ongoing | — | Severe immunodeficiency if thymus absent (DiGeorge syndrome) |
| Childhood (2–12 yr) | GH drives linear growth (epiphyseal plates open); IGF-1 mediates growth | Gigantism (GH excess before plate fusion): proportionate height increase | Pituitary dwarfism (GH deficiency): proportionate short stature, normal intelligence |
| Puberty (10–16 yr) | GnRH pulses increase → FSH/LH rise → gonadal maturation; GH/IGF-1 peak drives pubertal growth spurt; Thymus begins to involute | Precocious puberty (can be due to adrenal androgen excess in CAH) | Delayed puberty (hypogonadism) |
| Reproductive age (adult) | HPG axis active; menstrual cycle (females); spermatogenesis (males); PTH-calcitonin maintain calcium | Acromegaly (GH excess after plate fusion); Cushing's; Graves' disease; Conn's syndrome; phaeochromocytoma | Addison's; Hypothyroidism (myxoedema); Type 1 DM (any age); Hypoparathyroidism (tetany) |
| Old age | Oestrogen falls (menopause) → FSH/LH rise; GH falls; thymus completely involuted; insulin resistance increases | — | Osteoporosis (low oestrogen → less calcitonin activity → bone loss); Type 2 DM (insulin resistance); reduced T-cell immunity |