Adrenals and Thyroid Lecture Notes Flashcards

A set of practice flashcards covering the anatomy, hormones, functions, and clinical imbalances of the thyroid, parathyroid, and adrenal glands.

Thyroid Gland

Located in the anterior neck, inferior to the larynx, it is composed of two lateral lobes connected by an isthmus and is the largest purely endocrine organ in the body.

Follicle cells

Endocrine cells of the thyroid gland that produce thyroglobulin.

Parafollicular cells (C-cells)

Cells located between adjacent thyroid follicles that produce calcitonin to regulate calcium homeostasis.

Thyroxine ($T_4$)

A thyroid hormone derived from iodinated thyroglobulin consisting of 2 tyrosine and 4 iodine atoms.

Triiodothyronine ($T_3$)

The active form of thyroid hormone consisting of 2 tyrosine and 3 iodine atoms.

Thyroglobulin

A large glycoprotein scaffold stored in colloid that provides the tyrosines necessary for thyroid hormone synthesis.

Sodium-iodide symporter (NIS)

Also known as the 'iodine trap,' this is an integral membrane protein on the outer plasma membrane of thyroid epithelial cells that takes up iodide ($I^-$) from the blood.

Thyroid peroxidase (TPO)

An enzyme in the apical plasma membrane of thyroid epithelial cells that catalyzes the iodination of tyrosines ('organification of iodide') and the synthesis of $T_3$ or $T_4$.

Calorigenic effect

The action of thyroid hormone in increasing the body's metabolic rate and heat production.

Calcitonin

A hormone released in response to high blood $Ca^{2+}$ levels that inhibits osteoclast activity and stimulates $Ca^{2+}$ uptake into the bone matrix.

Parathyroid Glands

4 to 8 small glands embedded in the posterior aspect of the thyroid that regulate calcium levels through the secretion of parathyroid hormone (PTH).

Chief cells

The most prevalent cells in the parathyroid gland which sense calcium levels and secrete parathyroid hormone (PTH).

Parathyroid hormone (PTH)

The main regulator of $Ca^{2+}$ homeostasis that increases blood $Ca^{2+}$ by stimulating bone breakdown, enhancing kidney reabsorption, and activating Vitamin D.

Oxyphil cells

A cell type in the parathyroid gland which contains more calcium-sensing receptors (CaR) than chief cells and is thought to be derived from chief cells.

Adrenal medulla

The inner core of the adrenal gland composed of nervous tissue that synthesizes and secretes catecholamines (epinephrine and norepinephrine).

Adrenal cortex

The outer three layers of the adrenal gland made of glandular tissue that synthesize and secrete corticosteroids.

Catecholamines

Hormones including epinephrine (80%) and norepinephrine (20%) released by the adrenal medulla in response to the sympathetic nervous system ('fight or flight').

Zona glomerulosa

The thin, dark-staining outermost zone of the adrenal cortex that produces mineralocorticoids such as aldosterone.

Zona fasciculata

The thick middle zone of the adrenal cortex that stains paler and produces glucocorticoids such as cortisol.

Zona reticularis

The thin inner zone of the adrenal cortex that produces androgens such as androstenedione and dehydroepiandrosterone.

Aldosterone

A mineralocorticoid that increases renal retention of $Na^+$ and excretion of $K^+$, leading to increased blood volume and blood pressure.

Cortisol

A glucocorticoid released in response to ACTH that stimulates gluconeogenesis, mobilizes fatty acids, and has anti-inflammatory effects.

Congenital iodine deficiency syndrome (cretinism)

A condition caused by insufficient iodine during development, resulting in intellectual disability, short stature, and a thick tongue.

Myxedema

A condition of hypothyroidism in adulthood characterized by a low metabolic rate, lethargy, and mental sluggishness.

Goiter

An enlarged thyroid gland caused by inadequate iodine intake, where colloid accumulates because thyroid hormone cannot be synthesized or secreted.

Graves’ Disease

An autoimmune condition where antibodies mimic TSH, leading to hypersecretion of thyroid hormones, increased metabolic rate, and exophthalmos.

Exophthalmos

A symptom of Graves’ disease where the eyes protrude due to edematous tissues behind the eyes.

Addison’s Disease

A condition caused by the hyposecretion of glucocorticoids (cortisol and aldosterone), leading to low blood glucose, low blood pressure, and fatigue.

Cushing’s Syndrome

A condition caused by prolonged exposure to high levels of cortisol, often due to excessive medication (prednisone) or a pituitary adenoma.

General Adaptation Syndrome (GAS)

A stress response occurring in four phases: alarm, resistance, exhaustion, and death, initiated by the hypothalamus.