thyroid and adrenal glands

thyroid gland anatomy

2 (connected) lobes just inferior to thyroid cartilage (highly vascularized)

follicle

• smallest functional unit

• fluid (colloid) filled sphere lined by simple cuboidal epithelial cells

• important for synthesis/release of thyroid hormone

parafollicular cells

• synthesis/release of calcitonin hormone

• not involved in production of thyroid hormone

calcitonin

• released when too much Ca2+ in blood

• inhibit osteoclasts

• increase excretion of calcium by the kidney

• prevent absorption of calcium by digestive system

thyroid hormones

derived from the AA tyrosine

iodine

essential dietary element

use iodine to make thyroid hormone with tyrosine

forms of thyroid hormone

1. T₄: thyroxine; 4 iodine atoms; most abundant form of thyroid hormone

2. T₃: triiodothyronine; 3 iodine atoms; most biologically active form of thyroid hormone (body can do more things)

thyroid hormone pathway

1. thyroid gland releases T₄ into bloodstream

2. T₄ travels in the blood to target cells like kidney and liver

3. enzymes from target cells remove iodine from T₄ (deiodination)

4. converts T₄ into T₃

5. T₃ is the more active form so it produces a stronger effect inside cells

synthesis of thyroid hormones

1. iodide atoms are transported from blood into follicular cell (active transport and TSH dependent)

2. thyroid peroxidase converts iodide ions into iodine atoms and pairs it with thyroglobulin

3. T₃ and T₄ are formed within thyroglobulin

4. endocytosis of thyroglobulin

5. lysosomal degradation of thyroglobulin to release T₃ and T₄

6. diffusion of thyroid hormones from follicle cells into capillaries of plasma

7. transport in plasma using carrier proteins

thyroglobulin

globular protein secreted by follicle cells into colloid; abundance of tyrosine

regulation of thyroid hormones

1. hypothalamus releases TRH

2. TRH enters hypophyseal portal system and is distributed to thyrotrophs

3. anterior pituitary does the synthesis/release of TSH

4. TSH enters bloodstream to thyroid glands

5. thyroid glands produce thyroid hormones

6. thyroid hormones are released into bloodstream and have effects on target cells or negative feedback on hypothalamus and anterior pituitary

actions of thyroid hormones

affect almost every cell in the body → fast, strong, short increase in rate of cellular respiration

1. increase metabolic rate (heat production) → increase in body temp; more true for children

2. increase in heart rate and blood pressure

3. stimulate red blood cell formation → increase in oxygen delivery

4. accelerate turnover of minerals in bone → osteoblasts/osteoclasts: influence on bone remodeling

thyroid hormone receptors within a cell

1. cytoplasmic receptors - storage

2. mitochondria receptors - increase in ATP production (increase in thyroid hormones)

3. nucleus - increase in gene transcription

• sodium potassium pump, glycolytic enzymes (upregulation)

hypothyroidism

deficient thyroid hormones (not enough ATP); typically bc not enough iodine to make it

1. hypothalamus releases TRH

2. stimulates anterior pituitary to release TSH

3. this makes thyroid gland to produce T₃ and T₄ but there’s a low concentration

thyroid gland isn’t producing enough thyroid hormones → no negative feedback to tell the brain to stop

goiter

common effect of hypothyroidism

abnormal enlargement of thyroid

hyperthyroidism

excess thyroid hormone

1. hypothalamus thinks there’s too much thyroid hormones and stops releasing TRH

2. causes anterior pituitary to stop production of TSH

3. thyroid stimulating antibody mimics the effects of TSH and is attached to TSH receptor

4. thyroid gland makes excess thyroid hormones

grave’s disease (antibody activates TSH receptor), goiter, increased T₃ and T₄

grave’s ophthalmopathy

excess antibody binds to CT around the eye → create excess fat in the eye → eyes bulge

parathyroid hormones

4 small glands embedded on posterior surface of thyroid

collection of parathyroid principle cells

parathyroid principle cells

secrete PTH in response to decreased blood Ca²⁺ levels

opposite of calcitonin

effects of parathyroid principle cells

• stimulates osteoclasts

• enhances reabsorption of Ca²⁺ by kidney

• stimulates formation of calcitriol by kidney

  • promotes absorption of Ca²⁺ from digestive system

calcium’s physiological roles

normal plasma levels: ~8.8-10.2 mg/dL

• nerve and muscle excitation

• muscle contraction

• blood coagulation

• bone mineral balance

• intracellular signaling

anatomy of adrenal glands

retroperitoneal above each kidney

outer cortex: corticosteroids (2 dozen steroid hormones)

inner medulla: epinephrine and norepinephrine

divisions of outer cortex

1. zona glomerulosa: most superficial

2. zona fasiculata: thickest layer

3. zona reticularis: deepest; closest to adrenal medulla

hormones of the adrenal cortex

mineralcorticoids, glucocorticoids, adrenal androgens

all are steroids and derived from cholesterol

mineralcorticoids

zona glomerulosa

regulation of sodium and potassium levels in ECF

aldosterone

primary mineralcorticoid

released in response to low levels of Na+

reabsorb Na+ by forming urine in kidney, sweat glands, and salivary glands at the expense of K+

glucocorticoids

zona fasciculata

regulation of carbohydrate levels in ECF

cortisol, corticosterone

glucocorticoids

speed up rate of glucose synthesis (gluconeogenesis) and glycogen formation

anti-inflammatory properties

adrenal androgens

zona reticularis

“weak” androgens

useful as precursors for the production of estrogen and testosterone by other tissues

influence muscle mass and sex drive in adult women

hypothalamic pituitary adrenal (HPA) axis

• release of CRH is increased by stressors

• inhibition of release of CRH is initiated by cortisol (negative feedback loop)

chronic stress → chronic high levels of cortisol → desensitize receptor cells in the brain → continued release of CRH and continued production of cortisol