Chapter 16: Thyroid Gland

Where is the thyroid gland located?

Anterior neck on the trachea, just below the larynx

What does the thyroid gland control?

Metabolism

What connects the two lateral lobes of the thyroid gland?

The isthmus

What are the spherical units in the thyroid gland called?

Follicles

What do follicular cells produce?

Thyroglobulin

What does a colloid contain?

Thyroglobulin and iodine

• precursor to thyroid hormone

What is the function of parafollicular cells in the thyroid?

Produce the hormone calcitonin

What is the body's major metabolic hormone?

Thyroid hormone (TH)

The TH effects ____ cell in the body.

Every

The biochemical structure of the TH is found in what two forms?

T4 (thyroxine) and T3 (triiodothyronine)

T₄ (thyroxine) contains two ____ molecules with ____ bound iodine atoms. and must be converted to _____ at ____ level.

tyrosine, four, T₃, tissue

T₃ (triiodothyronine) contains two _____ molecules with ___ bound iodine atoms.

tyrosine and three

Both T₃ and T₄ are what type of hormones?

Iodine containing amine hormones

Which form of thyroid hormone is more abundant in the blood?

T4 (thyroxine)

Which thyroid hormone form is active at the tissue level?

T3 (triiodothyronine)

What must happen to T4 at the tissue level?

It must be converted to T3.

Are T3 and T4 water-soluble or lipid-soluble?

lipid soluble

Where do lipid soluble hormones enter and what do they trigger?

Enter target cell and binds to intracellular receptors within nucleus

• triggering transcription

What are three major effects of thyroid hormone on the body?

• Increases BMR and heat production

• Regulates tissue growth and development

• Maintains blood pressure

How does the TH regulate tissue growth and development?

Critical for normal skeletal and NS development & reproductive capabilities

How does the TH maintain blood pressure?

Increases adrenergic receptors in blood vessels

What does the thyroid gland store?

The thyroid hormone in the follicle lumen of the until triggered by TSH to release

What triggers the release of thyroid hormone from the follicle lumen?

Thyroid-stimulating hormone (TSH)

What is the first step in thyroid hormone synthesis?

Thyroglobulin is synthesized and discharged into the follicle lumen.

What happens in the second step of TH synthesis (iodide trapping)?

Iodide ions are actively transported into follicular cells and released into the follicle colloid

What is the third step in TH synthesis (oxidation)?

Iodide is oxidized to iodine (I2) by removing electrons.

Iodine is attached to ___ and mediated by ______

Tyrosine, Peroxidase enzymes

What is monoiodotyrosine (MIT)?

Formed if only one iodine attaches

What is diiodotyrosine (DIT)?

formed if two iodine attached

What links together to form T₃ and T₄?

Iodinated tyrosine

How is T3 formed in the thyroid gland?

By linking one MIT and one DIT

How is T4 formed in the thyroid gland?

By linking two DIT molecules

What happens after the thyroid hormone precursors are formed? (step 6)

The colloid is endocytosed by follicular cells, forming a vesicle.

What occurs inside the vesicle containing the colloid? (step 7)

The vesicle fuses with a lysosome, and enzymes cleave T3 and T4 from thyroglobulin.

What happens to T3 and T4 after they are cleaved from thyroglobulin?

They are secreted into the bloodstream.

Which form is mostly secreted by the thyroid gland?

T4

Which form must be converted for biological activity at the target tissue?

T4 must be converted to T3

How are T3 and T4 transported in the blood?

By thyroxine-binding globulins (TBGs)

Which thyroid hormone is more active at target cells?

T3 is 10 times more active than T4

What must happen to T4 for it to be active at tissues?

It must be converted to T3 by removal of one iodine atom

What regulates thyroid hormone release?

Negative feedback loop

What happens when thyroid hormone levels fall?

TSH (thyroid-stimulating hormone) is released

What inhibits TSH release?

• Rising levels of TH

• Somatostatin (GHIH)

• Dopamine

• Increased cortisol

• High iodide levels

What hormone can overcome negative feedback during pregnancy or exposed to cold?

TRH (thyrotropin-releasing hormone)

What is myxedema?

Hyposecretion of TH in adults

Symptoms of myxedema include:

• Low metabolic rate

• Feeling cold

• Mental sluggishness and lethargy

• Constipation

• Thick/dry skin

• Edema and puffy eyes

What causes a goiter in hypothyroidism?

Lack of iodine

• ↓ TH and ↑ TSH → thyroid enlarges with unviable thyroglobulin

Why is TH important in early childhood?

Critical for normal growth and brain development

What is the most common cause of hypersecretion? (hyperthyroidism)

Graves' disease (an autoimmune disorder)

What happens in Graves' disease?

Body makes abnormal antibodies that mimic TSH to stimulate TH release by binding to thyroid follicular cells

Symptoms of Graves' disease include:

• High metabolic rate

• Weight loss

• Sweating and nervousness

• Rapid, irregular heartbeat

• Exophthalmos (protruding eyes due to fibrous edematous tissue)

Treatments for Graves' disease include:

Surgical removal of the thyroid or Radioactive iodine to destroy thyroid cells

Where is calcitonin produced?

Parafollicular (C) cells of the thyroid

What triggers calcitonin release?

High blood calcium levels

Calcitonin is an antagonist to what hormone?

Parathyroid hormone (PTH)

Does calcitonin have a major role in adult humans under normal conditions?

No, it has no known significant role at normal levels

What does calcitonin do at pharmacological (high) doses?

• Inhibits osteoclast activity

• Prevents Ca²⁺ release from bone

• Stimulates calcium uptake into bone matrix