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Questions and Answers
What is the primary role of the Na-I symporter (NIS) in thyroid hormone synthesis?
What is the primary role of the Na-I symporter (NIS) in thyroid hormone synthesis?
Which hormone stimulates the thyroid gland to increase blood flow, size and activity?
Which hormone stimulates the thyroid gland to increase blood flow, size and activity?
What is the immediate precursor to the hormones T3 and T4?
What is the immediate precursor to the hormones T3 and T4?
Which of the following is NOT a direct effect of TSH release on the thyroid gland?
Which of the following is NOT a direct effect of TSH release on the thyroid gland?
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What is the role of thyrotropin-releasing hormone (TRH) in thyroid hormone regulation?
What is the role of thyrotropin-releasing hormone (TRH) in thyroid hormone regulation?
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What is the primary function of the Na-Iodide Symporter (NIS) in thyroid hormone synthesis?
What is the primary function of the Na-Iodide Symporter (NIS) in thyroid hormone synthesis?
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Where is thyroglobulin (TG) synthesized?
Where is thyroglobulin (TG) synthesized?
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What is the immediate precursor to iodide before it enters the thyroid follicular cells?
What is the immediate precursor to iodide before it enters the thyroid follicular cells?
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How does thyroglobulin reach the colloid?
How does thyroglobulin reach the colloid?
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Which transport protein is directly involved in establishing the electrochemical gradient necessary for NIS function?
Which transport protein is directly involved in establishing the electrochemical gradient necessary for NIS function?
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Which of the following scenarios would most likely result in a false positive for hypothyroidism?
Which of the following scenarios would most likely result in a false positive for hypothyroidism?
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What is the primary role of the thyroid hormone at the cellular level?
What is the primary role of the thyroid hormone at the cellular level?
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If a patient has a condition causing an increase in free T3 and T4 breakdown, what would you expect to observe alongside this in test results?
If a patient has a condition causing an increase in free T3 and T4 breakdown, what would you expect to observe alongside this in test results?
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What is the first step in the secretion of thyroid hormone from colloid to blood?
What is the first step in the secretion of thyroid hormone from colloid to blood?
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Which of these situations would result in more free thyroid hormone available?
Which of these situations would result in more free thyroid hormone available?
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Which of the following accurately describes the state of T3/T4 circulation in the blood?
Which of the following accurately describes the state of T3/T4 circulation in the blood?
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Which of the following is NOT a thyroid hormone binding protein?
Which of the following is NOT a thyroid hormone binding protein?
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Which of the following best describes the metabolic effect of thyroid hormone?
Which of the following best describes the metabolic effect of thyroid hormone?
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Which of the given proteins has the highest binding capacity for thyroid hormones?
Which of the given proteins has the highest binding capacity for thyroid hormones?
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Which of the following thyroid hormones is the most biologically active?
Which of the following thyroid hormones is the most biologically active?
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Following receptor binding, how do thyroid hormones primarily increase target cellular activity?
Following receptor binding, how do thyroid hormones primarily increase target cellular activity?
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What is the expected effect of thyroid hormone imbalance on hair growth?
What is the expected effect of thyroid hormone imbalance on hair growth?
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What is the primary role of thyroid hormone in metabolism?
What is the primary role of thyroid hormone in metabolism?
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What is the general effect of thyroid hormones on the basal metabolic rate (BMR)?
What is the general effect of thyroid hormones on the basal metabolic rate (BMR)?
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How do thyroid hormones generally affect the nervous system?
How do thyroid hormones generally affect the nervous system?
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What is the general effect of thyroid hormones on the gastrointestinal tract?
What is the general effect of thyroid hormones on the gastrointestinal tract?
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What is the general effect of thyroid hormones on the heart?
What is the general effect of thyroid hormones on the heart?
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What is the effect of thyroid hormones on fetal or juvenile growth and development?
What is the effect of thyroid hormones on fetal or juvenile growth and development?
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What is the primary role of thyroglobulin (TG) in thyroid hormone synthesis?
What is the primary role of thyroglobulin (TG) in thyroid hormone synthesis?
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Which enzyme is responsible for the oxidation of iodide (I-) to iodine (I2) in the thyroid gland?
Which enzyme is responsible for the oxidation of iodide (I-) to iodine (I2) in the thyroid gland?
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Which transporter is responsible for moving iodide from the follicular cell into the colloid?
Which transporter is responsible for moving iodide from the follicular cell into the colloid?
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What is the primary function of the Sodium-Iodide Symporter (NIS) in thyroid hormone production?
What is the primary function of the Sodium-Iodide Symporter (NIS) in thyroid hormone production?
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What is the 'organification' step in thyroid hormone synthesis?
What is the 'organification' step in thyroid hormone synthesis?
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An increase in which of the following is not a direct effect of T3 on the digestive system?
An increase in which of the following is not a direct effect of T3 on the digestive system?
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During the 'coupling reaction', which enzyme is responsible for facilitating the formation of T3 and T4?
During the 'coupling reaction', which enzyme is responsible for facilitating the formation of T3 and T4?
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MIT and DIT are precursor molecules in thyroid hormone synthesis. What does MIT stand for?
MIT and DIT are precursor molecules in thyroid hormone synthesis. What does MIT stand for?
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How does T3 primarily affect fat storage in the body?
How does T3 primarily affect fat storage in the body?
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What is the primary component of the colloid in the thyroid follicle?
What is the primary component of the colloid in the thyroid follicle?
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Which of the following is a known effect of T3 on the cardiovascular system?
Which of the following is a known effect of T3 on the cardiovascular system?
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How does T3 impact the kidneys?
How does T3 impact the kidneys?
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Which of the following describes the correct order of steps in thyroid hormone synthesis?
Which of the following describes the correct order of steps in thyroid hormone synthesis?
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What is the effect of normal T3 levels on muscle protein?
What is the effect of normal T3 levels on muscle protein?
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What is primarily transported using the Pendrin channel in thyroid follicular cells?
What is primarily transported using the Pendrin channel in thyroid follicular cells?
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In a euthyroid sick patient, which thyroid hormone level is most likely to be suppressed?
In a euthyroid sick patient, which thyroid hormone level is most likely to be suppressed?
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If a patient's blood work shows a low TT4, low fT4, and high TSH, what condition is most likely?
If a patient's blood work shows a low TT4, low fT4, and high TSH, what condition is most likely?
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T3 plays a crucial role in fetal and neonatal development by influencing which of the following?
T3 plays a crucial role in fetal and neonatal development by influencing which of the following?
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Study Notes
Thyroid Physiology 2.0
- Meghan Fick DVM, MS, DACVECC is the presenter
- Clinical Assistant Professor of Emergency and Critical Care
- Office: VMC 2107
- Email: [email protected]
Review: Abbreviations
- TRH = Thyrotropin releasing hormone
- TSH = Thyroid stimulating hormone
- NIS = Na-I Symporter
- TG = Thyroglobulin
- TPO = Thyroid peroxidase
- MIT = Mono-iodotyrosine
- DIT = Di-iodotyrosine
Review: Terminology
- Iodine (I2) = two iodine atoms
- Iodide (I-) = one iodine atom
- T3 = tri-iodothyronine
- T4 = thyroxine
Thyroid Hormone Formation Prep
- Step 0.1: Iodine ingested as I- (iodide)
- Step 0.2: I- absorbed through the GI tract, enters bloodstream
- Step 0.3: I- reaches the thyroid gland, which is highly vascular
- Step 0.4: Low T3 and T4 levels stimulate hypothalamus to secrete TRH
- Step 0.5: TRH travels through hypothalamic-hypophyseal portal circulation to the adenohypophysis
- Step 0.6: TSH is secreted from the adenohypophysis into the bloodstream
Thyroid Hormone Synthesis
- Two major components: Tyrosine, Part of thyroglobulin stored in colloid. TG is a HUGE protein with lots of tyrosine residues. Iodine (I2)
- Derived from dietary iodide (I-)
- Important elements to the synthesis of thyroid hormone
Thyroid Hormone Synthesis Steps (Diagrammatic)
- Iodide transported into colloid via Pendrin channel
- Iodide oxidized in the colloid
- TPO adds I onto TG at tyrosine residues
- Coupling Reaction (catalyzed by TPO)
- TG, T3, and T4 are components involved in synthesis
Secretion (Diagrammatic)
- Pseudopod endocytoses colloid containing thyroglobulin into the follicular cell
- Vesicle containing TG fuses with a lysosome which cleaves T3 and T4
- T3 and T4 diffuse out of the cell and TG is recycled
T3 and T4 at the cells
- Penetrate cell membranes (lipophilic)
- TR = intra-nuclear thyroid receptor
- TR affinity (T3) >> T4
- Will increase or decrease gene transcription
- Proteins, enzymes, structural, transport components will be affected by the change
- Elicit biological effect
Thyroid Hormone Function
- Impacts all nucleated cells in the body
- Generally increases cell function and metabolism
- Main function is to control the metabolic rate (how much energy the body is using)
T3 and Metabolism
- Overall catabolic effects (breakdown)
- Increased tissue metabolism (thermogenesis)
- Increase oxygen consumption
- Increase energy availability and usage
- Glucose utilization
- Lipid mobilization
- Balanced protein synthesis and breakdown
T3 and Haircoat
- Regulates: Development hair follicle stem cells, Follicle growth and function, Rate of hair growth and cycling
- Hormone imbalance can lead to: Slowed or stopped hair growth, Hair loss, Failure of hair regrowth
T3 and Nervous System
- Stimulatory!
- Increase mental alertness
- Shortened nerve reaction times
- Increased responsiveness to stimuli
T3 and Digestive System
- Increases appetite
- Increases GI motility
- Increases GI secretion
- Increases glucose absorption
T3 and Digestive System- Actions on Fat
- Mobilizes free fatty acids
- Reduces fat storage
- Decreases plasma cholesterol
- Increases biliary excretion
- Decreases plasma triglycerides
T3 and Digestive System- Actions on Muscle
- Normal hormone levels = balanced protein synthesis and breakdown
- Contraction strength & relaxation time
- Energy utilization
- Muscular repair following injury
T3 and Cardiovascular System
- Sensitizes myocytes to catecholamines
- Epinephrine, norepinephrine
- Increased adrenergic receptor expression
- Increases cardiac output
- Increased heart rate
- Increased contractility
- Increased stroke volume
T3 and Kidneys
- Promotes vasodilation
- Skin = heat dissipation
- Heart and muscle = increases blood flow
- RAAS activation
- Renin-angiotensin-aldosterone system
- Increases blood volume
- Increases renal blood flow and GFR
T3 and Growth & Development
- Essential for fetal and neonatal development
- Brain maturation
- Axonal growth and myelin sheath formation
- Muscle development
- Bone growth
- Stimulates secretion of and works with growth hormone
Euthyroid Sick
- Suppression of thyroid hormone concentrations due to concurrent illness, stress, trauma or surgery.
- TT4 is most susceptible
- fT4 is suppressed with moderate to severe illness
- TSH is typically NORMAL
- Difficult to diagnose hypothyroid dog that is sick
- Stabilize, treat the underlying disease, re-assess thyroid panel following recovery
Additional Notes:
- 99% of circulating thyroid hormones are protein bound
- About 1% are FREE hormones (Free T4, Free T3) and Total T3/T4 = protein bound + free hormones
- Important binding proteins: Thyroxine-binding globulin (TBG), Transthyretin (TTR), Thyroxine-binding prealbumin (TBPA), Albumin
- T3 is ~10x more potent than T4
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Description
Test your knowledge on thyroid physiology with this quiz that covers important terms, hormone formation, and key abbreviations. Perfect for medical students or anyone interested in endocrinology. Get ready to dive into the details of thyroid function!
