Endocrine Part 4: Thyroid Gland Flashcards
Which of the following correctly describes the anatomical structure of the thyroid gland?
A) The thyroid gland consists of follicles that contain parafollicular cells, which secrete T3 and T4.
B) The thyroid gland is located in front of the neck and contains follicular cells that secrete calcitonin.
C) The thyroid gland is composed of follicular cells that secrete T3 and T4, while parafollicular cells secrete calcitonin.
D) The thyroid gland is positioned behind the trachea and produces TSH to regulate hormone release.
The thyroid gland consists of follicular cells responsible for secreting T3 and T4, while parafollicular cells secrete calcitonin. It is located in front of the neck, not behind the trachea.
What is the first step in the synthesis of thyroid hormones?
A) Coupling of iodotyrosines
B) Iodide uptake through the sodium-iodide symporter (NIS)
C) Iodination of tyrosine residues in thyroglobulin
D) Secretion of T3 and T4 into the bloodstream
Uptake of Iodine (from diet) by Na symporters)
What is the correct sequence of thyroid hormone synthesis?
A) Iodination of thyroglobulin → Iodide uptake → Coupling of iodotyrosines → Storage → Secretion
B) Coupling of iodotyrosines → Iodide uptake → Iodination → Thyroglobulin synthesis → Secretion
C) Iodide uptake → Thyroglobulin synthesis → Iodination → Coupling → Storage → Secretion
D) Iodide uptake → Coupling of iodotyrosines → Iodination → Storage → Secretion
Thyroid hormone synthesis begins with iodide uptake, followed by thyroglobulin synthesis (which contains tyrosine residues). Then, iodination of tyrosine occurs, forming MIT and DIT, which then couple to form T3 and T4. Finally, the hormones are stored and later secreted.
C
Which of the following is NOT a general property of thyroid hormones?
A) They are synthesized from tyrosine and iodine.
B) They are stored in thyroglobulin within thyroid follicles.
C) They circulate freely in the blood without binding to proteins.
D) T3 is more biologically active than T4.
thyroid hormones circulate bound to proteins (e.g., thyroid-binding globulin (TBG)), with only a small percentage in free form. T3 is more biologically active than T4, and the hormones are stored in thyroglobulin before secretion.
They circulate freely in the blood without binding to proteins.
Which of the following is NOT a physiological function of thyroid hormones?
A) Increasing basal metabolic rate (BMR)
B) Enhancing sympathetic nervous system activity
C) Increasing serum cholesterol levels
D) Promoting normal growth and development
Thyroid hormones decrease serum cholesterol levels by increasing cholesterol metabolism. They also increase BMR, enhance the sympathetic nervous system, and are essential for growth and development.
Why thyroid hormones enhance sympathetic nerve activity:
Thyroid hormones (especially T₃) increase the number and sensitivity of β-adrenergic receptors (especially β₁ receptors) in target tissues — like the heart, skeletal muscle, and adipose tissue.
C
Lecture notes: thyroid hormone physiological actions:
- acts on most tissues
- increases metabolism
- responsible for growth and developmet
- reuglates body tempurature (due to metabolism)
How is thyroid hormone secretion regulated?
A) T3 and T4 stimulate TRH and TSH release in a positive feedback loop.
B) High levels of T3 and T4 inhibit TRH and TSH release via negative feedback.
C) TSH directly converts T4 into T3 in the bloodstream.
D) Iodine directly inhibits the release of TRH from the hypothalamus.
✅ Answer: B
Rationale: Thyroid hormone secretion is regulated by negative feedback. High levels of T3 and T4 inhibit the release of TSH from the anterior pituitary and TRH from the hypothalamus, maintaining hormonal balance.
Which of the following correctly describes the effects of too much or too little thyroid hormone?
A) Hyperthyroidism leads to weight gain, cold intolerance, and lethargy, while hypothyroidism causes weight loss, heat intolerance, and rapid heart rate.
B) Hyperthyroidism causes increased metabolism, weight loss, and heat intolerance, while hypothyroidism leads to slow metabolism, weight gain, and cold intolerance.
C) Hypothyroidism results in an overactive sympathetic nervous system, while hyperthyroidism leads to decreased BMR and myxedema.
D) Both hyperthyroidism and hypothyroidism cause weight gain and lethargy.
✅ Answer: B
Rationale: Hyperthyroidism increases metabolism, leading to weight loss, heat intolerance, and a rapid heart rate. Hypothyroidism slows metabolism, resulting in weight gain, cold intolerance, and lethargy.
What are thw two most important molecules for synthesizing thyroid hormones?
A. Tryrosine
B. Theorine
C. Iodine
B. Na+
Tyrosine and Iodine
How are MIT and DIT different from T4 and T3?
MIT and DIT are intermediates during thyroid hormone synthesis
Try + one iodine = MIT
Tyr + 2 iodine = DIT
T3= MIT + DIT
T4= DIT + DIT
T4 and T3 are the active hormone end products of thyroid hormone synthesis
T4 is a prohormone (stored in the body) and T3 is the active form for T4.
Why is t4 more abundant than t3, depsite t3 being the active form? And how does t3 become active
Although T3 is more potent than T4, T4 is a prohormone
T4 is converted to T3 by peripheral cells and liver cells, then T3 binds to TBP (thyroid binding protein) and bind to nuclear receptors
Which of the following is NOT a metabolic effect of T3?
A) Increased basal metabolic rate (BMR) and oxygen consumption
B) Increased carbohydrate absorption and utilization
C) Increased serum LDL cholesterol levels
D) Increased fat breakdown and cholesterol metabolism
T3 decreases serum LDL cholesterol by increasing cholesterol metabolism. It also increases BMR, oxygen consumption, carbohydrate utilization, and fat breakdown, making metabolism more efficient.
Lecture notes: Actions of T3 on metabolism are
- increased canabolism and anabolism
- increased Basal metabolic rate with increased oxygen, leading to more consumption and increased heat production
- increased carbohhydrate absorption and utilization
- increased protien breakdown
- increased fat breakdown
- increased cholesterol breakdown
- decreased serum cholesterol
How do the thyroid hormones affect the cardiovascular system, potentiation, and reproductive system?
Cardio:
Increased HR and contractility, increases blood pressure
Potentiation in sympathetic nervous system: increased potentiation, longer fight or flight mode
Reproductive system: maintains normal functions and fertility
(te binds to adrenergic receptors, causing fight or flight responses)
Autoimmune disease that causes antibodies to bind to TSH receptors, which causes overactivity of the gland
Graves disease (overactivity of the gland, hyperthyroidism)
Antibodies mimic TSH
A disorder of the thyroid gland that is associated with increased exophthalmos, goitre, BMR, weight loss,
Graves disease
An autoimmune disorder where thyroid peroxidase antibodies destroy thyroid glands to block hormone synthesis
Hashimoto’s thyroiditis (hypothyroidism, underactivity of thyroid glands)
A case of hypothyroidism associated with adults:
Myxedema (swelling of face)
A case of hypothyroidism associated in children, affecting growth:
Cretinism
Why can goitre occur in both hypothyroidism and hyperthyroidism?
A) In hyperthyroidism, excessive TSH stimulates thyroid enlargement, while in hypothyroidism, low T3 and T4 cause increased TSH secretion, leading to thyroid growth.
B) Goitre only occurs in hypothyroidism due to iodine deficiency, causing the thyroid gland to swell.
C) In hyperthyroidism, antibodies attack thyroid peroxidase, while in hypothyroidism, antibodies attack the TSH receptor, both causing glandular swelling.
D) Goitre is only seen in Graves’ disease due to an autoimmune attack on the TSH receptor.
Answer A
In hyperthyroidism (grave’s disease), the thyroid gland will enlarge due to excessive stimulation of TSH receptors by autoantibodies.
Hypothyroidism has a lack of iodine, causing low T3 and T4 levels, leading to increased TSH.
Both diseases cause increased TSH, causing goitre formation
Why is TSH measurement the best single screening test for thyroid function?
A) TSH levels directly regulate iodine uptake, making them the best indicator of thyroid activity.
B) TSH levels change before measurable changes in T3 and T4, providing an early indicator of thyroid dysfunction.
C) TSH is produced by the thyroid gland, so its levels directly reflect thyroid hormone production.
D) TSH remains constant regardless of thyroid hormone levels, making it a reliable long-term marker.
Answer: B
Rationale: TSH measurement is the best screening test because changes in TSH levels occur before detectable changes in T3 and T4. Since TSH is regulated by negative feedback from T3 and T4, even slight thyroid dysfunction alters TSH levels first, making it a highly sensitive early marker for both hyperthyroidism and hypothyroidism.
Which of the following is correct (C) or not correct (NC)?
a. Goitre happens only due to hyperthyroidism.
b. Levels of TSH measurement is a good indication for diseases involving hyper- or hypo- secretion of the thyroid gland.
A) also occurs with hypothyroidism
B) yes
Besides hypothyroidism and hoitre, what else can iodine defficiencies cause?
Prevent brain damage and disability development in early life (pregnancy and baby)
how does the body store thyroid hormonesÉ where do thyroid hormones go to be releasedÉ
thyroid hormones are stored in the colloid with thyroglobulin
release happens when thyrogoblulin is removed by endocytosis and t3 and t4 are secreted from the follicle cell into the blood and mostly travel in the blood by a protein
