Endocrine Glands and Organs: Labeled Overview

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

The endocrine system is the collection of ductless glands and scattered hormone-secreting cells that release chemical messengers directly into the bloodstream to regulate metabolism, growth, reproduction, stress responses, and fluid balance. A labeled endocrine system maps each gland to its hormones, its target tissues, and the feedback loops that keep hormone concentrations within a narrow working range.

This matters because hormones act at very low concentrations and small shifts produce large effects. A thyroid gland that underproduces hormone slows metabolism across every organ. An adrenal cortex that overproduces cortisol breaks down muscle and suppresses immunity. A pancreatic islet that stops releasing insulin produces rapid, life-threatening hyperglycemia. Understanding which gland makes which hormone, and how the brain monitors the result, is the foundation for interpreting endocrine laboratory results and for recognizing when a patient's signs point to a hormonal cause rather than an organ-specific disease.

The Endocrine Organ Map

Endocrine tissue in domestic animals falls into three anatomical patterns. Discrete glands (thyroid, adrenal, pituitary) are compact organs with a defined capsule and blood supply. Scattered cell clusters (pancreatic islets, gastrointestinal enteroendocrine cells) sit inside organs that have other primary functions. Diffuse single cells (some pulmonary and renal endocrine cells) release hormones without forming any visible structure.

The major endocrine system organs, listed from the central controller outward:

1. Hypothalamus. A region of the ventral diencephalon containing neurosecretory neurons. It releases releasing and inhibiting hormones into the hypophyseal portal system and sends oxytocin and antidiuretic hormone down axons to the posterior pituitary.
2. Pituitary gland (hypophysis). A two-lobed gland in the sella turcica. The anterior lobe (adenohypophysis) produces tropic hormones under hypothalamic control. The posterior lobe (neurohypophysis) stores and releases hypothalamic peptides.
3. Thyroid gland. A bilobed gland ventral to the trachea. Its follicles store thyroglobulin and produce thyroxine (T4) and triiodothyronine (T3). Its parafollicular C cells produce calcitonin in mammals.
4. Parathyroid glands. Small paired glands on or near the thyroid capsule. They secrete parathyroid hormone (PTH), the primary regulator of blood calcium.
5. Adrenal glands. Paired glands cranial to the kidneys. The outer cortex produces steroid hormones in three zones. The inner medulla produces catecholamines.
6. Pancreatic islets. Clusters of cells scattered through the exocrine pancreas. Beta cells make insulin, alpha cells make glucagon, and delta cells make somatostatin.
7. Gonads. Ovaries and testes produce sex steroids and peptide hormones that drive reproductive cycles and secondary sex characteristics.
8. Placenta. A transient endocrine organ that produces progesterone, estrogens, and species-specific placental hormones to maintain pregnancy.

Two additional tissues deserve a place on any labeled endocrine system. Bone acts as a dynamic endocrine organ, and bone-derived factors together with systemic hormones and neuroendocrine cues shape skeletal stem and progenitor cell behavior during maintenance and repair [1]. Adipose tissue secretes adipokines such as omentin-1, which modulates tropic hormone secretion and signaling in anterior pituitary cells, linking metabolism to reproduction [2].

The Hypothalamic-Pituitary Unit

The hypothalamus and pituitary function as a single control unit. Hypothalamic neurons secrete releasing hormones into a capillary bed at the median eminence. These short portal veins carry the releasing hormones directly to the anterior pituitary, where they bind specific cells and trigger or inhibit hormone release.

The anterior pituitary produces growth hormone, thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin. Each has a dedicated hypothalamic releasing hormone, and most have an inhibiting hormone as well.

The posterior pituitary is neural tissue, not glandular tissue. It stores oxytocin and antidiuretic hormone (also called vasopressin) that were synthesized in hypothalamic cell bodies and transported down axons. Release is triggered by nerve signals, not by releasing hormones.

The Adrenal Gland in Cross Section

The adrenal cortex is divided into three zones with distinct steroid output. The zona glomerulosa, outermost, produces mineralocorticoids, chiefly aldosterone, under the control of the renin-angiotensin-aldosterone system. The zona fasciculata, the widest zone, produces glucocorticoids, chiefly cortisol, under ACTH control. The zona reticularis, innermost, produces adrenal androgens.

The adrenal medulla is functionally a modified sympathetic ganglion. Chromaffin cells release epinephrine and norepinephrine directly into the bloodstream in response to sympathetic stimulation.

Adrenal cortical organization shows measurable species and sex differences. Spatial transcriptomic profiling of the mouse adrenal cortex identified ten distinct cell populations, with sex differences concentrated in the inner cortical zones, and the female-predominant X-zone was confirmed as a specialized progesterone-catabolizing compartment marked primarily by Akr1c18 [3]. Female adrenal glands in that study showed greater intercellular communication complexity, with 435 predicted interactions versus 369 in males [3].

Hormone Table: Source, Target, and Primary Action

Negative Feedback: Two Worked Axes

Negative feedback is the core control principle of endocrinology. A hormone's own downstream effect shuts off the signal that produced it. Two axes illustrate the pattern.

The Thyroid Axis: TRH to TSH to T4 and T3

The hypothalamic-pituitary-thyroid axis begins when hypothalamic neurons release TRH into the portal circulation. TRH binds thyrotropes in the anterior pituitary and triggers TSH release. TSH binds receptors on thyroid follicular cells and drives every step of thyroid hormone production: iodine uptake, thyroglobulin synthesis, hormone coupling, and release of T4 and T3 into the blood.

T4 is the main circulating product, and most of it is converted to the active T3 in peripheral tissues. Both hormones act on nearly every cell to set basal metabolic rate.

The feedback loop closes at two levels. Rising T4 and T3 concentrations inhibit TRH release from the hypothalamus and TSH release from the pituitary. When thyroid output falls, the inhibition weakens, TRH and TSH rise, and the gland is driven harder. This is why a low T4 with a high TSH points to primary thyroid disease, while a low T4 with a low or normal TSH points to a pituitary or hypothalamic problem.

The axis is sensitive to external disruption. Plasticizers such as phthalates and bisphenols interfere with thyroid hormone synthesis, transport, receptor signaling, iodine homeostasis, and feedback regulation, contributing to thyroid autoimmunity and broader endocrine imbalance [4]. Experimental work in zebrafish showed that probiotic supplementation helped maintain hypothalamic-pituitary-thyroid axis homeostasis and affected thyroid hormone levels and their specific receptors after toxic exposure [5]. The clinical lesson is that thyroid test results reflect the whole axis, not just the gland.

The Cortisol Axis: CRH to ACTH to Cortisol

The hypothalamic-pituitary-adrenal (HPA) axis runs in parallel. Hypothalamic neurons release CRH, which drives ACTH release from the anterior pituitary. ACTH binds the zona fasciculata and stimulates cortisol synthesis and secretion. Cortisol then raises blood glucose through gluconeogenesis, moderates inflammation, and maintains vascular responsiveness to catecholamines.

Cortisol feeds back negatively on both the hypothalamus and the pituitary. When cortisol is high, CRH and ACTH fall. When cortisol is low, both rise.

This axis is the body's main stress response system, and it interacts with signals beyond the classic hormones. Growth and differentiation factor 15 (GDF-15) is a stress-responsive hormone that signals through a brainstem-restricted receptor and shows a dynamic interplay with the HPA axis, with evidence suggesting it can activate the axis and that downstream glucocorticoid signaling may contribute to metabolic, immune, and musculoskeletal changes [6]. The gut also feeds into this system. Stress-related dysregulation of the brain-gut axis alters gastrointestinal motility, visceral sensation, mucosal barrier integrity, and central affective circuits through the HPA axis, the autonomic nervous system, immune signaling, and gut microbial ecology [7]. Probiotic strains within the Lactobacillus and Bifidobacterium genera can attenuate stress reactivity and regulate HPA axis hyperactivity [8].

Exogenous glucocorticoids suppress the axis by the same mechanism. A case report described a patient who developed glucocorticoid-induced adrenal insufficiency after abruptly stopping a supplement that contained undisclosed dexamethasone, confirmed by a low random cortisol, an inadequate cosyntropin stimulation response, and an inappropriately normal ACTH [9]. A cross-sectional study of skin-whitening cream users found significantly lower mean morning serum cortisol (4.25 ± 2.23 µg/dL) compared with non-users (7.85 ± 3.54 µg/dL), with 42% of users below 5 µg/dL, suggesting reduced HPA axis activity from chronic topical corticosteroid absorption [10]. In horses and foals, critical illness-related corticosteroid insufficiency is a transient HPA axis dysfunction that produces inadequate cortisol responses during severe stress from endotoxemia and sepsis [11].

Positive Feedback: Oxytocin in Parturition

Not every endocrine loop is negative. Oxytocin during parturition is the standard example of positive feedback, where the output amplifies the original signal rather than shutting it down.

Fetal descent and cervical stretch trigger sensory signals that reach the hypothalamus and cause oxytocin release from the posterior pituitary. Oxytocin binds receptors on uterine smooth muscle and produces stronger contractions. Stronger contractions push the fetus further, stretching the cervix more, which triggers more oxytocin release. The loop escalates until the fetus is delivered, at which point the stimulus disappears and oxytocin release falls.

The same hormone drives milk letdown through a similar reflex. Suckling stimulates oxytocin release, oxytocin contracts myoepithelial cells around mammary alveoli, and milk flows. This loop is neuroendocrine rather than hormonal feedback, but the amplification principle is the same.

How the Endocrine System Is Studied and Observed

Endocrine testing in practice follows three approaches.

Basal hormone measurement. A single blood sample establishes whether a hormone is present at an appropriate concentration. Timing matters because many hormones are pulsatile or follow a daily rhythm. Cortisol, for example, peaks in the morning in most species.

Dynamic testing. A stimulating or suppressing agent is given and the response is measured. The cosyntropin stimulation test assesses adrenal reserve by measuring cortisol before and after synthetic ACTH. The dexamethasone suppression test assesses whether the axis can be shut down. Dynamic testing is more informative than a single value because it tests the axis as a system, and recent advances in dynamic testing and point-of-care hormone tests are improving assessment in equine patients [11].

Imaging and histology. Ultrasound, radiography, and cross-sectional imaging localize enlargements and masses. Histology confirms cell type. Endocrine glands have characteristic architectural patterns that identify them on section, and reference atlases of endocrine histology provide the standard images for comparison [12].

A fourth approach is emerging. Directed differentiation of human pluripotent stem cells has produced pancreatic beta cells advanced enough for clinical investigation, and the same three-tier framework of cell identity, in vivo survival, and functional delivery is now being applied to thyroid, parathyroid, pituitary, and adrenal cells [13]. Cross-species single-cell studies of pancreatic and intestinal endocrine progenitors have identified conserved and distinct gene regulatory networks during lineage allocation [14].

Comparative Species Notes

Cats and dogs differ in adrenal steroid profiles. The adrenal cortex of both species produces cortisol as the principal glucocorticoid, but the intermediate steroid pathways and the relative proportions of cortisol, corticosterone, and androgens differ between them. This is why reference intervals for adrenal steroids are species-specific and why a panel validated in dogs cannot be read against feline intervals. The spatial organization of the cortex also matters. In the mouse, spatial transcriptomics showed that sex differences concentrate in the inner cortical zones and that the X-zone is a progesterone-catabolizing compartment [3]. Comparative adrenal anatomy across domestic species follows the same three-zone plan with species variation in zone width and lipid content.

Birds use ultimobranchial bodies instead of parathyroid C cells. In mammals, the thyroid gland contains parafollicular C cells that produce calcitonin. In birds, the calcitonin-producing cells sit in separate ultimobranchial bodies near the parathyroid glands, and the thyroid contains no C cells. Avian calcium regulation therefore involves a different anatomical arrangement of the same hormonal players, with the ultimobranchial body supplying calcitonin and the parathyroids supplying PTH.

Ruminants have unique placental hormones. The ruminant placenta produces interferon-tau during early pregnancy as the maternal recognition signal, and it secretes pregnancy-associated glycoproteins and placental lactogens that support the corpus luteum and prepare the mammary gland. These hormones are not found in the same form in carnivores or equids. Placental endocrine function in general is a maternal-placental-fetal system. During pregnancy, cortisol supports placental function and fetal maturation, and the developmental significance of any exposure depends on timing, magnitude, duration, placental glucocorticoid regulation, and fetal susceptibility [15].

Quick Review

• The hypothalamus controls the anterior pituitary through releasing hormones delivered by a portal blood system.
• The posterior pituitary stores hypothalamic peptides and is neural, not glandular, tissue.
• The adrenal cortex has three zones: glomerulosa for aldosterone, fasciculata for cortisol, reticularis for androgens.
• Negative feedback means the end hormone shuts off its own upstream signal, as in the TRH-TSH-T4/T3 and CRH-ACTH-cortisol axes.
• Positive feedback means the output amplifies the signal, as in oxytocin during parturition and milk letdown.
• Birds put calcitonin-producing cells in ultimobranchial bodies rather than in the thyroid.
• Exogenous steroids suppress the HPA axis and can cause adrenal insufficiency when stopped abruptly.

The main control logic of the endocrine system can be summarized as a single decision path.

flowchart TD
    A[Hypothalamus detects need] --> B[Releasing hormone to portal blood]
    B --> C[Anterior pituitary releases tropic hormone]
    C --> D[Target gland secretes end hormone]
    D --> E[End hormone acts on target tissues]
    E --> F{End hormone level adequate}
    F -->|Yes| G[Inhibit hypothalamus and pituitary]
    F -->|No| H[Continue stimulation]
    G --> I[Hormone level returns to set point]
    H --> C

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Clinical Relevance, Limitations and Common Mistakes

Endocrine disease often presents with signs that point away from the gland. A dog with hypothyroidism may present for weight gain, coat changes, or lethargy. A cat with hyperthyroidism may present for weight loss despite a good appetite. A horse with pituitary pars intermedia dysfunction may present for a long hair coat that fails to shed. Recognizing the endocrine pattern early shortens the diagnostic path.

Interpretation errors are common. A single cortisol value cannot diagnose or exclude adrenal insufficiency because cortisol is pulsatile and stress-responsive, which is why dynamic testing exists [11]. A normal ACTH with a low cortisol does not exclude secondary adrenal insufficiency, as the supplement case demonstrated [9]. Thyroid results must be read as an axis, because thyroid-disrupting chemicals act at multiple points including synthesis, transport, receptor signaling, and feedback regulation [4].

The HPA axis is also a target of everyday clinical exposures. Topical corticosteroids absorbed from skin products can suppress morning cortisol [10]. Undisclosed steroids in supplements can do the same [9]. Gut health influences the axis through the brain-gut signaling network [7], and microbiome-modulating interventions can reduce HPA axis hyperactivity [8].

Individual patients require veterinary assessment. Reference intervals, test protocols, and normal responses vary by species, age, reproductive status, and laboratory method, so no general overview can substitute for a diagnosis made with a veterinarian who knows the patient.

Frequently Asked Questions

What are the main endocrine system organs?

The main endocrine system organs are the hypothalamus, pituitary gland, thyroid gland, parathyroid glands, adrenal glands, pancreatic islets, gonads, and placenta. Bone and adipose tissue also contribute endocrine signals.

What is the difference between the anterior and posterior pituitary?

The anterior pituitary is glandular tissue that makes its own hormones under hypothalamic control. The posterior pituitary is neural tissue that stores and releases oxytocin and antidiuretic hormone made in the hypothalamus.

How does negative feedback work in the thyroid axis?

Rising T4 and T3 concentrations inhibit TRH release from the hypothalamus and TSH release from the pituitary. When thyroid output falls, that inhibition weakens and TSH rises to drive the gland harder.

Why is oxytocin an example of positive feedback?

Oxytocin release causes uterine contractions that push the fetus against the cervix, which triggers more oxytocin release. The cycle escalates until delivery removes the stimulus.

Do birds have the same endocrine glands as mammals?

Birds have a hypothalamus, pituitary, thyroid, parathyroids, adrenals, pancreatic islets, and gonads. Their calcitonin-producing cells sit in separate ultimobranchial bodies rather than inside the thyroid gland.

Can stopping a steroid cause adrenal problems?

Yes. Exogenous glucocorticoids suppress the HPA axis, and abrupt withdrawal can leave the adrenal glands unable to mount an adequate cortisol response. This has been documented with prescribed steroids and with undisclosed steroids in supplements.

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