IVRI Anatomy
A free veterinary anatomy learning platform for B.V.Sc., M.V.Sc., DVM and veterinary medicine students worldwide, developed at the Veterinary Anatomy Section, ICAR-Indian Veterinary Research Institute, Bareilly—covering VCI/MSVE and international university curricula through regional anatomy, histology, embryology, comparative biomechanics, clinical notes, quizzes and revision.
THYROID GLAND (Glandula thyreoidea):
Located beside the cranial cervical trachea. Exact level, shape and isthmus development vary among cattle, horse, pig and carnivores and should be learned species-specifically.
Histology: Built of follicles — spherical structures lined by follicular cells (synthesise T3/T4 stored as thyroglobulin in colloid lumen). Between follicles: parafollicular ('C') cells secreting calcitonin.
Hormones:
• T4 (thyroxine) + T3 (triiodothyronine) — regulate basal metabolic rate, growth, development. Iodine essential.
• Calcitonin — from C-cells; lowers blood Ca²⁺ by inhibiting osteoclasts.
Vasculature: Cranial thyroid artery (from common carotid) + caudal thyroid artery (often inconstant).
Innervation: cervical sympathetic + parasympathetic via vagus.
PARATHYROID GLANDS:
Domestic species generally possess external and internal parathyroid tissue, but number and topography vary. External glands lie separate from thyroid parenchyma and may be located at a distance (notably in cattle); internal glands are embedded in or closely applied to thyroid tissue. Cranial/caudal developmental origin should be described separately.
Hormone: Parathormone (PTH) — RAISES blood Ca²⁺ by:
1. Mobilising Ca²⁺ from bone (osteoclast activation).
2. Increasing Ca²⁺ reabsorption in kidney.
3. Activating vitamin D → increases gut Ca²⁺ absorption.
Antagonist of calcitonin.
| OX | Thyroid lobes elongated. Iodine-deficient pastures cause endemic goiter in calves. |
| HORSE | Thyroid easily palpable in some horses. Equine thyroid neoplasia common in geriatric. |
| DOG | Hypothyroidism is a common canine endocrine disorder. Naturally occurring hyperthyroidism is uncommon in dogs and much more characteristic of older cats. |
| CAT | Hyperthyroidism from thyroid adenoma — most common feline endocrine disease in old cats. |
Explore the hidden mechanics of why structures are present, absent or modified across domestic and wildlife species—and how each anatomical design supports movement, function and clinical practice.
Explanation goes here.