125 Questions
What is the correct way thyroid hormones exert their physiological effects?
By acting on target tissues in their free form
Which plasma protein has the largest capacity to bind T4?
Thyroxine-binding globulin (TBG)
What is the primary form of thyroid hormone found in plasma?
T4 bound to TBG
What percentage of circulating T4 is converted to T3 in adult humans?
Not specified in the text
What is the effect of prolonged TSH stimulation on the thyroid gland?
It leads to thyroid enlargement, known as a goiter
What is the primary site of deiodination of T4?
Liver
What is the primary function of thyroid hormones in the body?
Maintenance of optimal metabolism in tissues
Which hormone stimulates the secretion of thyroid-stimulating hormone (TSH) from the anterior pituitary?
Thyrotropin-releasing hormone (TRH)
How are thyroid hormones synthesized in the thyroid gland?
Thyroglobulin is synthesized in the thyroid cells and secreted into the colloid, where it undergoes iodination
What is the function of thyroid peroxidase in thyroid hormone synthesis?
Oxidation and iodination of tyrosine residues on thyroglobulin
What is the role of thyroglobulin in thyroid hormone synthesis?
Synthesis and secretion into the colloid for iodination
What is the source of iodine for thyroid hormone synthesis?
Iodine absorbed from the diet
What is the outcome of inadequate thyroid hormone synthesis in the thyroid gland?
Hypothyroidism
Which of the following is NOT a step in thyroid hormone synthesis?
Deiodination of thyroglobulin in the thyroid gland
What is the role of thyrotropin-releasing hormone (TRH) in thyroid hormone synthesis?
Stimulation of thyroid-stimulating hormone (TSH) secretion from the anterior pituitary
What is the primary source of iodine for thyroid hormone synthesis?
Dietary iodine
How does thyroid peroxidase contribute to thyroid hormone synthesis?
By oxidizing and iodinating tyrosine residues on thyroglobulin
What is the effect of thyrotropin-releasing hormone (TRH) on the thyroid gland?
Stimulation of thyroid hormone secretion
What is the effect of removing the pituitary gland on thyroid function?
It leads to thyroid function suppression.
What is the primary regulator of circulating levels of pituitary TSH?
Thyroid hormones
Which of the following is true about transthyretin?
It has a shorter half-life compared to thyroxine-binding globulin (TBG).
What is the primary site of deiodination of T4?
Liver
What is the result of prolonged stimulation of the thyroid gland by TSH?
Thyroid gland enlargement (goiter).
What percentage of circulating T4 is bound to albumin?
None of the above
What is the primary regulator of circulating levels of pituitary TSH?
Hypothalamus
Where does deiodination of T4 primarily occur?
Liver
What is the outcome of removing the pituitary gland on thyroid function?
Thyroid function inhibition
What is the effect of prolonged TSH stimulation on the thyroid gland?
Thyroid gland enlargement (goiter)
What is the primary form of thyroid hormone bound to albumin in plasma?
T4
What percentage of circulating T4 is converted to T3 in adult humans?
Unknown percentage
What is the primary function of thyroid hormones in the body?
Maintenance of optimal metabolism in tissues
Which hormone stimulates the secretion of thyroid-stimulating hormone (TSH) from the anterior pituitary?
Thyrotropin-releasing hormone (TRH)
How are thyroid hormones synthesized in the thyroid gland?
Thyroglobulin is synthesized in the thyroid cells and secreted into the colloid, where it undergoes iodination
What is the function of thyroid peroxidase in thyroid hormone synthesis?
Oxidation and iodination of tyrosine residues on thyroglobulin
What is the role of thyroglobulin in thyroid hormone synthesis?
Iodination of tyrosine residues on thyroglobulin
What is the primary source of iodine for thyroid hormone synthesis?
Dietary iodine
What is the primary function of oxytocin in mammals?
Helping with contractions of the uterus and milk ejection during breastfeeding
What is the role of vasopressin in the body?
Regulation of water balance
What is the intermediate lobe of the pituitary responsible for?
Regulation of skin pigmentation
What percentage of circulating growth hormone activity is in the bound form?
50%
What type of cell in the anterior pituitary secretes prolactin?
Lactotropes
What is the function of corticotropes in the anterior pituitary?
Secretion of adrenocorticotropic hormone (ACTH)
What is the typical basal plasma growth hormone level in adult humans?
Less than 3 ng/mL
Which hormone is secreted by the gonadotropes in the anterior pituitary?
Follicle-stimulating hormone (FSH) and luteinizing hormone (LH)
Where is growth hormone primarily metabolized?
Liver
What is the approximate half-life of circulating growth hormone in humans?
6-20 minutes
What is the function of the plasma protein bound to growth hormone?
To provide a reservoir of growth hormone
What is the effect of growth hormone on the pancreas?
It increases the ability of the pancreas to respond to insulinogenic stimuli
What is the name of the growth factor that stimulated the incorporation of sulfate into cartilage?
Sulfation factor
How does growth hormone bind to its receptor?
Through two binding sites, producing a homodimer
What is the primary circulating somatomedin in humans?
Insulin-like growth factor I
What is the effect of insulin on protein metabolism?
It has an anabolic effect
How many related relaxin isoforms are found in humans?
Two
What is the extension of the A chain in insulin-like growth factors?
D domain
What is necessary for receptor activation?
Dimerization
What signaling pathway does growth hormone activate?
JAK2–STAT pathway
What type of kinases does JAK2 belong to?
Cytoplasmic tyrosine kinases
What happens to the epiphysial plates in young animals treated with growth hormone?
They widen
What is the result of prolonged treatment with growth hormone in young animals?
Gigantism
What is the result of an overabundance of growth hormone in adults?
Acromegaly
Where are the mRNAs for IGF-I and IGF-II found?
In the liver
What stimulates the secretion of IGF-I after birth?
Growth hormone
What is the role of IGF-II in the fetus?
It is involved in growth and development
What happens to organs in fetuses that overexpress IGF-II?
They develop out of proportion to the rest of the body
Where is the gene for IGF-II expressed in adults?
In the choroid plexus and meninges
What is the primary function of oxytocin in mammals?
Helping with contractions of the uterus and milk ejection during breastfeeding
What type of cells in the anterior pituitary secrete ACTH?
Corticotropes
What is the role of vasopressin in the body?
Acting as a water regulator
What is the function of gonadotropes in the anterior pituitary?
Secreting FSH and LH
What is the intermediate lobe of the pituitary responsible for?
Secreting hormonally active molecules of POMC
What type of cell in the anterior pituitary secretes growth hormone?
Somatotropes
What is the primary source of hGH-V and hCS?
Placenta
What percentage of circulating growth hormone activity is bound to a plasma protein?
50-60%
How is the growth hormone receptor structured?
Large extracellular domain, transmembrane domain, and cytoplasmic portion
What is the half-life of circulating growth hormone in humans?
6-20 min
What is the function of the plasma protein bound to growth hormone?
Provides a reservoir of growth hormone
How does growth hormone bind to its receptor?
Two binding sites, producing a homodimer
What is the location of the growth hormone-hCS cluster on human chromosomes?
The long arm of human chromosome 17
What is the function of the glycosylation of growth hormone?
Posttranslational modification of the hormone
How many genes are present in the growth hormone-hCS cluster on human chromosome 17?
Five
What is the name of the variant form of growth hormone encoded by the hGH-V gene?
Variant growth hormone
What is the pituitary hormone that is composed of α and β subunits?
Human chorionic gonadotropin (hCG)
What is the result of the complex mixture of growth hormone peptides in the circulation?
Difficulty in assaying for each species separately
What is the primary pathway activated by growth hormone?
JAK2–STAT pathway
What is the effect of growth hormone on young animals?
Stimulates growth
What is the result of prolonged treatment with growth hormone in animals?
Gigantism
What happens to the epiphysial plates in response to growth hormone?
They widen
What is the result of an overabundance of growth hormone in humans?
Acromegaly
What is the effect of hypophysectomy on growth in young animals?
Inhibits growth
What is the indirect effect of growth hormone on the pancreas?
It increases the ability of the pancreas to respond to insulinogenic stimuli
What is the name of the somatomedin initially isolated because it stimulated the incorporation of sulfate into cartilage?
Sulfation factor
What is the relation between IGF-I and insulin?
IGF-I has a similar structure to insulin but lacks a C chain separation
What is the function of somatomedins in relation to growth hormone?
Somatomedins are stimulated by growth hormone and promote growth
What is unique about the structure of IGF-I and IGF-II compared to insulin?
Their C chains are not separated
What is the effect of growth hormone on protein metabolism?
It has a protein anabolic effect
What is the main function of the pituitary gland?
To coordinate control of many downstream endocrine glands
Which hormone secreted by the anterior pituitary acts on the breast?
Prolactin
What is the source of almost all of the blood supply to the anterior pituitary?
Portal hypophysial vessels
What is the role of the tropic hormones secreted by the anterior pituitary?
To stimulate secretion of hormonally active substances by other endocrine glands
How many separate endocrine organs does the pituitary gland contain in some species?
Three
What is the location of the pituitary gland?
In a pocket of the sphenoid bone
Which hormone helps with contractions of the uterus and milk ejection during breastfeeding?
Oxytocin
What type of cells in the anterior pituitary secrete ACTH?
Corticotropes
What is the function of vasopressin in the body?
Regulation of water balance
Which type of cells in the anterior pituitary secrete growth hormone?
Somatotropes
Which structure is rudimentary in humans?
Intermediate lobe of the pituitary
What is the function of melanocyte-stimulating hormones (MSHs) in the intermediate lobe of the pituitary?
Regulation of skin pigmentation
What percentage of circulating growth hormone activity is in the bound form?
50%
What is the primary site of growth hormone metabolism?
Liver
What is the half-life of circulating growth hormone in humans?
6-20 min
What is the function of the plasma protein bound to growth hormone?
To provide a reservoir of growth hormone
How does growth hormone bind to its receptor?
Through two binding sites, producing a homodimer
What is the daily growth hormone output in adults?
0.2-1.0 mg/d
What is the primary function of JAK2 in the JAK2–STAT pathway?
To phosphorylate STATs
What is the effect of hypophysectomy on growth in young animals?
It inhibits growth
What is the result of prolonged treatment of animals with growth hormone?
It leads to gigantism
What is the effect of an overabundance of growth hormone in humans with closed epiphyses?
It leads to acromegaly
What is the effect of growth hormone on the sizes of most viscera?
It increases their sizes
What is the primary role of STATs in the JAK2–STAT pathway?
To activate various genes in the nucleus
What is the effect of growth hormone on β cells of the pancreas?
It increases their ability to respond to insulinogenic stimuli
What is the primary function of somatomedins in the body?
They have a protein anabolic effect
What is the relationship between insulin and somatomedins?
They have similar structures, but somatomedins have an additional domain
What is the name of the growth factor that stimulated the incorporation of sulfate into cartilage?
Sulfation factor
How many circulating somatomedins are there in humans?
Multiple, but only two are principal
What is the effect of growth hormone on cartilage?
It stimulates cartilage formation
Study Notes
Thyroid Hormones
- Primary function of thyroid hormones: Maintenance of optimal metabolism in tissues
- Synthesis of thyroid hormones: Thyroglobulin is synthesized in the thyroid cells and secreted into the colloid, where it undergoes iodination
- Function of thyroid peroxidase: Oxidation and iodination of tyrosine residues on thyroglobulin
- Physiological effects of thyroid hormones: Acting on target tissues in their free form
- Primary form of thyroid hormone found in plasma: T4 bound to thyroxine-binding globulin (TBG)
- Plasma protein with the largest capacity to bind T4: Thyroxine-binding globulin (TBG)
- Percentage of circulating T4 converted to T3 in adult humans: Not specified
- Deiodination of T4: Primarily occurs in the liver
- Prolonged TSH stimulation: Can lead to thyroid enlargement, known as a goiter
Growth Hormone
- Primary function of growth hormone: Regulation of growth and development
- Secretion of growth hormone: Stimulated by thyrotropin-releasing hormone (TRH)
- Effects of growth hormone:
- Increases the ability of the pancreas to respond to insulinogenic stimuli
- Promotes growth by stimulating the incorporation of sulfate into cartilage and collagen formation
- Stimulates the production of somatomedins (insulin-like growth factor I (IGF-I) and IGF-II)
- Somatomedins: Polypeptide growth factors secreted by the liver and other tissues
- Effects of somatomedins:
- Stimulate the growth of cartilage and bone
- Increase the size of viscera
- Play a role in the growth of the fetus before birth
- Receptors: Growth hormone receptor is a 620-amino-acid protein with a large extracellular portion, a transmembrane domain, and a large cytoplasmic portion
- Binding and metabolism:
- Approximately 50% of the circulating pool of growth hormone activity is in the bound form
- Basal plasma growth hormone level: Normally less than 3 ng/mL
- Half-life of circulating growth hormone: 6–20 min
- Daily growth hormone output: 0.2–1.0 mg/d in adults
Pituitary Gland
-
Cell types in the anterior pituitary:
- Somatotropes: Secrete growth hormone
- Lactotropes: Secrete prolactin
- Corticotropes: Secrete ACTH
- Thyrotropes: Secrete TSH
- Gonadotropes: Secrete FSH and LH
-
Posterior pituitary:
- Stores oxytocin and vasopressin in the termini of neurons
- Releases oxytocin and vasopressin into the bloodstream
- Oxytocin: Helps with contractions of the uterus and milk ejection during breastfeeding
- Vasopressin: Regulates water balance in the body### Dimerization and Receptor Activation
-
Dimerization is essential for receptor activation
-
Growth hormone has two domains that can bind to its receptor, and when it binds to one receptor, the second binding site attracts another, producing a homodimer
-
Dimerization is necessary for receptor activation
Growth Hormone Effects
- Growth hormone has widespread effects in the body
- Activates many different intracellular signaling cascades
- Activates the JAK2–STAT pathway
- JAK2 is a member of the Janus family of cytoplasmic tyrosine kinases
- STATs (signal transducers and activators of transcription) are a family of cytoplasmic transcription factors
Effects on Growth
- In young animals, growth is inhibited by hypophysectomy and stimulated by growth hormone
- Chondrogenesis is accelerated, and as the cartilaginous epiphysial plates widen, they lay down more bone matrix at the ends of long bones
- Stature is increased
- Prolonged treatment of animals with growth hormone leads to gigantism
- When the epiphyses are closed, linear growth is no longer possible
- An overabundance of growth hormone produces the pattern of bone and soft tissue deformities known in humans as acromegaly
- Sizes of most of the viscera are increased
Somatomedins
- Somatomedins are polypeptide growth factors secreted by the liver and other tissues
- First isolated as sulfation factor, which stimulated the incorporation of sulfate into cartilage
- Also stimulated collagen formation, and its name was changed to somatomedin
- There are a variety of different somatomedins, and they are members of an increasingly large family of growth factors
- Principal circulating somatomedins are insulin-like growth factor I (IGF-I, somatomedin C) and IGF-II
- These factors are closely related to insulin, except that their C chains are not separated and they have an extension of the A chain called the D domain
The Pituitary Gland
- The pituitary gland, or hypophysis, lies in a pocket of the sphenoid bone at the base of the brain
- A coordinating center for control of many downstream endocrine glands
- Anterior pituitary secretes:
- Thyroid-stimulating hormone (TSH, thyrotropin)
- Adrenocorticotropic hormone (ACTH)
- Luteinizing hormone (LH)
- Follicle-stimulating hormone (FSH)
- Prolactin
- Growth hormone
- Posterior pituitary consists predominantly of nerves that have their cell bodies in the hypothalamus
- Stores oxytocin and vasopressin in the termini of these neurons, to be released into the bloodstream
Cell Types in the Anterior Pituitary
- Five types of secretory cells have been identified in the anterior pituitary by immunocytochemistry and electron microscopy:
- Somatotropes, which secrete growth hormone
- Lactotropes (also called mammotropes), which secrete prolactin
- Corticotropes, which secrete ACTH
- Thyrotropes, which secrete TSH
- Gonadotropes, which secrete FSH and LH
Test your knowledge of thyroid hormones and their functions in the body. Learn about the regulation of metabolism, calcium levels, and more. Explore the role of the pituitary gland and hormones involved in thyroid function.
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