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Physiology 2 > Jan 15 - Thyroid Gland > Flashcards

Jan 15 - Thyroid Gland Flashcards

1
Q

Describe the thyroid gland

A

It consists of two lobes of endocrine tissue joined in the middle by narrow portion of gland

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2
Q

What are the two types of cells that make up the thyroid gland?

A

Follicular cells

C cells

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3
Q

Describe follicular cells

A

They are arranged into hollow spheres
They form the functional unit called a follicle
They are responsible for the production of thyroid hormones thyroxine (T4) and triiodothyronine (T3)
The lumen is filled with colloid, which serves as extracellular storage site for thyroid hormones

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4
Q

What are the two names for the thyroid hormone T4?

A

Thyroxine or tetraiodothyronine

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5
Q

What is the difference between T3 and T4 (besides the number of iodine molecules)?

A

T4 is the major form in terms of production

T3 is metabolically active (T4 is a prohormone)

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6
Q

What is the role of C cells?

A

They secrete the peptide hormone calcitonin

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7
Q

What are the basic ingredients of thyroid hormones?

A

Tyrosine (synthesized in sufficient amounts by the body)

Iodine (obtained from dietary intake)

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8
Q

What does an iodine deficiency cause?

A

Goitre

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9
Q

Where does synthesis of the thyroid hormones take place?

A

All steps occur on the thyroglobulin molecules within the colloid

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10
Q

What are the steps of thyroid hormone synthesis?

A

Tyrosine-containing thyroglobulin is exported from follicular cells into colloid by exocytosis
Thyroid captures iodine from blood and transfers it into colloid by iodine pump.
Within colloid, iodine attaches to tyrosine. The attachment of one iodide to tyrosine yields monoiodotyrosine (MIT). The attachment of iodides to tyrosine yields diiodotyrosine (DIT).
Coupling process occurs between iodinated tyrosine molecules to form thyroid hormones. The coupling of MIT and DIT yields T3. The coupling of two DITs yields T4

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11
Q

Where are thyroid hormones stored?

A

Thyroid hormones remain in the colloid until they are split off and secreted. Usually there is enough thyroid hormones stored to supply a body’s need for several months (it takes several months before symptoms as a result of thyroid problems arise).

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12
Q

Describe the secretion of thyroid hormones

A

When the body needs thyroid hormones, the follicular cells phagocytize thyroglobulin-laden colloid. These pseudopods are integrated with lysozymes. This process frees T3 and T4 to diffuse across the plasma membrane and into the blood.

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13
Q

What are the effects of thyroid hormones?

A

They are the main determinant of basal metabolic rate. They influence synthesis and degradation of carbohydrate, fat and protein. They increase target-cell responsiveness to catecholamines. They increase heart rate and force of contraction. They are essential for normal growth. They play a crucial role in the normal development of nervous system

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14
Q

How is the hypothalamus-pituitary-thyroid axis regulated? Why is it important?

A

It is regulated by negative-feedback system between hypothalamic TRH, anterior pituitary TSH, and thyroid gland T3 and T4. The feedback loop maintains thyroid hormones relatively constant

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15
Q

What are the causes of hypothyroidism?

A

Primary failure of thyroid gland
Secondary to a deficiency of TRH, TSH or both
Inadequate dietary supply of iodine

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16
Q

Why is it important to determine what causes hypothyroidism?

A

You don’t want to give someone TSH if the problem is that the thyroid has stopped working

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17
Q

What are symptoms of hypothyroidism?

A

Sleepiness, weight gain, always cold, problems with memory, problems forming words

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18
Q

What is cretinism?

A

It results from hypothyroidism from birth. It’s quite severe. Babies are tested at birth to ensure they have sufficient thyroid hormones

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19
Q

What is myxedema?

A

It’s a term often used for severe hypothyroidism in adults. It frequently looks like someone has swollen hands and feet. Often their skin is coarse in appearance. Usually, replacement therapy with T4 is the solution (unless it’s dietary - iodine)

20
Q

When the cause of hypothyroidism is primary failure of the thyroid gland, what are the plasma concentrations of relevant hormones? Is there a goitre present?

A

There’s a low concentration of T3 and T4, and high concentration of TSH. There is a goitre present because TSH will cause the follicular cells to proliferate

21
Q

When the cause of hypothyroidism is secondary to hypothalamic or anterior pituitary failure, what are the plasma concentrations of relevant hormones? Is there a goitre present?

A

There is a low concentration of T3, T4, TSH and/or TRH. There is no goitre present

22
Q

When the cause of hypothyroidism is lack of dietary iodine, what are the plasma concentrations of relevant hormones? Is there goitre present?

A

There is a low concentration of T3, T4 and a high concentration of TSH. There is a goitre present because TSH will cause the follicular cells to proliferate

23
Q

What is the most common cause of hyperthyroidism? Describe it

A

Graves’ disease. It’s an autoimmune disease where the body erroneously produces thyroid-stimulating immunoglobins (TSI). Since TSI isn’t controlled by the negative-feedback loop, the thyroid is constantly being stimulated. It’s characterized by exopthalmos (the bulging of the eyes due to a build up of fluids behind the eye)

24
Q

How is hyperthyroidism treated?

A

Surgical removal of a portion of the over-secreting thyroid
Administration of radioactive iodine
Use of antithyroid drugs

25
Q

Why is important to closely regulate plasma calcium ion concentration?

A

To prevent changes in neuromuscular excitability. It also plays vital role in a number of essential activities (excitation-contraction coupling in cardiac and smooth muscle, stimulus-secretion coupling, maintenance of tight junctions between cells, clotting of blood)

26
Q

What is hypercalcemia? What is the result?

A

Too much calcium in the blood. It reduces excitability

27
Q

What is hypocalcemia? What is the result?

A

Too little calcium in the blood. It brings about overexcitability of nerves and muscles; severe overexcitability can cause fatal spastic contractions of respiratory muscles

28
Q

What used to be the risk of having a patient’s thyroid removed?

A

If the doctor removed the parathyroid glands, the patient would die of hypocalcemia

29
Q

What are the three hormones that regulate plasma concentration of calcium (and phosphate)?

A

Parathyroid hormone (PTH)
Calcitonin
Vitamin D

30
Q

What is the importance of carefully regulating the concentration of calcium and phosphate?

A

Calcium and phosphate can precipitate to make the bone hard. This needs to be carefully regulated so that calcium and phosphate don’t precipitate in the blood

31
Q

Describe parathyroid hormone (PTH)

A

It is secreted by parathyroid glands. It is a primary regulator of calcium ions (it raises free plasma calcium levels by its effects on the bones, kidneys and intestines). It is essential for life (prevents fatal consequences of hypocalcemia). It facilitates activation of vitamin D

32
Q

What is an osteoblast?

A

Single-nuclei cell that synthesizes bone

33
Q

What is an osteoclast?

A

A type of bone cell that reabsorbs bone tissue (it chews up bone and fragments of bone and releases calcium and phosphate)

34
Q

When are osteoblasts most active? When are osteoclasts most active?

A

When we are kids, new bone is being made. As we age, osteoblasts shut down (at about 30 years old) and then osteoclasts become more and more active

35
Q

What happens if osteoblasts secrete RANK ligand?

A

The RANK ligand binds with RANK receptor on macrophages and osteoclasts. This causes differentiation of macrophages into osteoclasts and the suppression of osteoclast apoptosis. The action of osteoclasts increases, the osteoclast action outpaces the osteoblast action and the bone mass decreases

36
Q

What happens if osteoblasts secrete osteoprotegerin (OPG)?

A

Osteoprotegerin binds with RANK ligand, making it unavailable for binding with RANK. This blocks the action of RANK, reducing osteoclast action. Osteoblast action outpaces osteoclast action and the bone mass increases

37
Q

What is the best advice for a patient looking to maintain strong bones?

A

Maintain weight bearing exercises as you age

38
Q

What happens if plasma calcium levels are too high?

A

The parathyroid gland is inhibited from secreting PTH. Thyroid C cells secrete calcitonin, which reduces plasma calcium levels (calcitonin only kicks in when calcium levels are really high)

39
Q

What happens if plasma calcium levels are too low?

A

Thyroid C cells are inhibited from secreting calcitonin. Parathyroid secretes PTH, which raises plasma calcium levels

40
Q

What is calcitonin? What is its role?

A

It is a hormone produced by C cells of thyroid gland. It works in a negative-feedback fashion (secreted in response to increase in plasma calcium concentration). It acts to lower plasma calcium levels by inhibiting activity of bone osteoclasts. It is unimportant except during hypercalcemia

41
Q

What is the role of vitamin D?

A

It stimulates calcium and phosphate absorption from intestines. It can be synthesized from cholesterol derivative when exposed to sunlight (which is often an inadequate source). The amount supplemented by dietary intake. It must be activated first by liver and then by kidneys before it can exert its effect on intestines

42
Q

How is vitamin D produced (and activated)?

A

The precursor in the skin or dietary vitamin D3 has a hydroxyl group added on by liver enzymes to give 25-OH-vitamin D3. Kidney enzymes (activated by low phosphate and low calcium) add on a second hydroxyl group to give 1,25-(OH)2-vitamine D3 (active form)

43
Q

What is the result of PTH hypersecretion (hyperparathyroidism)?

A

Hypercalcemia and hypophosphatemia

44
Q

What is the result of PTH hyposecretion (hypoparathyroidism)?

A

Hypocalcemia and hyperphosphatemia

45
Q

What is the result of a vitamin D deficiency?

A

In children, it causes rickets. In adults, it causes osteomalacia

Physiology 2 (19 decks)

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