Lecture 28: Thyroid Gland, Parathyroid Gland, Hormonal Regulation of Calcium

10/23 Online 3 Kahoots

Types of cells in thyroid gland

C cells (parafollicular cells) and Follicle cells

C cells (parafollicular cells)

Produce calcitonin in response to high calcium levels

Calcitonin

Inhibits osteoclasts and increase calcium ion excretion by kidney (pee it out)

Tone down calcium levels in blood

Thyroid Gland

Sits anterior to trachea → when it becomes enlarged you can feel it

Gland that secretes thyroid hormone and also regulates calcium levels

Acts like a lipid in terms of crossing the membranes

Follicle Cells

Release thyroid hormones (T4 and T3)

Where receptors are for TSH (produced and released into bloodstream depending on TSH and iodine levels

Follicle cavity is filled with sticky substance

T4

Thyroxine

has 4 iodide ions

Inactive form

T3

Triiodothyronine

three iodide ions

Active form

Thyroid Hormones 

Form of tyrosine (amino acid) with attached iodide ions 

How much of secretion is T4?

90%

Why is T4 important for the negative feedback loop?

This is the only hormone that the hypothalamus detects (not T3)

How does T4 become activated and what does it become?

Converted to T3 by enzymes in peripheral tissue, enzyme cleaves off one iodide ion leaving T3 the active form

What are thyroid hormones attached to in bloodstream?

Transport proteins creating large reserve supply of T4 and T3

What is the synthesis and release of TH controlled by?

TSH from adenohypophysis

Negative feedback loop example if T3 and T4

Homeostasis is disturbed by decreasing T3 and T4 concentration in blood or low body temperature

Receptors in hypothalamus that is response for T4 detection drops, secretes TRH

Anterior lop release TSH after being stimulated by TRH

Thyroid then releases T3 and T4

Homeostasis is restored

Function of thyroid hormones

produce strong, immediate, short-lasting increase in the rate of cellular metabolism and use of energy

Cross cell membrane and bind to intracellular receptors

• bind to mitochondria and increase rate of ATP production

• BInd to receptors activating genes that control energy utilization

• increases metabolism (body temp goes up)

What is thyroid hormone’s function in development?

Essential for normal development of skeletal, muscular, and nervous systems during childhood

Cretinism 

During growth periods you also secrete thyroid hormones because you need more energy to grow 

• excess thyroid hormone is produced in the process because you need to make more ATP 

Hyposecretion occurs → low thyroid hormone levels during growth that causes abnormal growth of all your systems 

Thyroid hormone is essential for normal what control in adults?

Metabolic

What are metabolic abnormalities due to thyroid abnormalities

Hypothyroidism

Hyperthyroidism

Goiter

Hypothyroidism

Common in females

Caused by autoimmune (Hasimatos’s) which makes antibodies that block TSH receptors → thyroid hormone levels drop → metabolic rate slows down → core has colder body temp (metabolism) → likely to gain weight, lethargic

Hyperthyroidism 

Autoimmune (Grave’s disease) causes, antibodies mimic TSH → thyroid gland overproduces TH → lose weight → high metabolic rate → high body temp → dangerous (“constant fever”) → can denature proteins 

Treatment for hyperthyroidism?

Thyroid gland removal or radioactive iodide, only thyroide can absorb/take iodide, radioactive iodide would destroy

Goiter

Enlarged thyroid

Lack of iodide (mainly iodized salt)

Essentially you make a bunch of precursor but no iodide to finish the process which leaves it lingering

Parathyroid Glands 

Four glands embedded in the posterior surface of the thyroid gland 

Secretes PTH

Parathyroid hormone (PTH)

Secreted by parathyroid gland when calcium concentration is lower than normal

Increases calcium ion concentration (does opposite of calcitonin)

What are some ways that PTH increase Ca2+ conc?

Stimulating osteoclasts

Inhibiting Osteoblasts

Decreasing Ca2+ excretion by kidneys

Stimulate formation of calcitriol by kidneys

How would stimulating osteoclasts increase Ca2+ conc in blood?

Osteoclasts are phagocytic bone cells that will dissolve bony matrix and release calcium into blood

How would inhibiting osteoblasts increase Ca2+ conc?

Osteoblasts are building cells

By stopping the remodeling of our bones we can use calcium that would’ve been used for our bones on other things 

How would you decrease calcium excretion by kidneys?

Less calcium would be peed out and reabsorbed back into blood

What is calcium levels maintained by?

Negative feedback loop

What does Ca2+ conc affect?

Nerve and muscle cell excitability

What is Ca2+ conc maintained by?

Negative feedback system involving both PTH and calcitonin

Bones and calcium

Storage of calcium ions (osteoclasts and osteoblasts)

Digestive tract and calcium

Absorption of calcium ions

• calcitriol

Kidney - Excretion of calcium ions

Calcitriol

Steroid hormone derived from vitamin D from the kidney, binds vitamin D to tissue in order to increase calcium levels