Thyroid Gland/Hormones Flashcards
T/F: The thyroid gland is highly vascular.
TRUE
Where is the thyroid gland located?
Just inferior to the larynx
Two lobes connected by an isthmus
Describe the function of the epithelial (follicle) cell in the synthesis of thyroid hormone.
- Iodide transported into cell through the basal side with Na
- Iodide diffuses across the follicle cell and into the colloid
- TG and thyroid peroxidase are synthesized and exocytosed into the colloid
What is the roll of the colloid in the synthesis of thyroid hormones?
- Iodide is oxidized by thyroid peroxidase and linked with tyrosine on TG to create MIT or DIT
- DITs and MITs combine to form either T3 or T4
- These T3s and T4s remain attached to TG and are stored in the colloid
Why is Iodide linked with TG?
Maintain concentration gradient so more Iodide diffuses across follicle cell and into colloid
Describe the organs and hormones involved in the thyroid axis.
- Hypothalamus releases thyrotropin releasing hormone (TRH)
- Ant pituitary releases thyroid stimulating hormone (TSH)
- Thyroid releases T3 and T4
Where are the receptors for TSH?
Follicle cells on the thyroid
Once TSH stimulates receptors on the follicle cells, describe the process for thyroid hormone secretion.
- Colloid droplets with TG + T3/T4 are pinocytosed into follicle cells
- Droplet fuses with lysosome with enzymes to cleave T3 and T4 from TG
- T3 and T4 released into cytoplasm and diffuse into capillaries
- Amino acids from TG recycled for new TG
Thyroid hormone is lipophilic. Most lipophilic hormones cannot be stored in vesicles. What allows for the storage of TH?
It is synthesized extracellularly (colloid) and is stored in a colloid droplet bound to TG
Which hormone (T3 or T4) is secreted in higher volumes?
T4
Which hormone (T3 or T4) is the most biologically active form of TH?
T3
What happens to the majority of T4 after it is secreted? Why is it secreted as T4 in the first place?
T4 is transferred to T3 in liver or kidneys
Secreting as T4 keeps Iodide concentration gradient low in colloid and high in blood. Insures there is always free iodide for TH synthesis
T/F: TSH serves only to stimulate TH synthesis.
FALSE
Also stimulates follicle cell metabolism and replication
T/F: Thyroid hormone exerts negative feedback on both TRH and TSH.
TRUE
In what cells are receptors for TH found?
Most cells of the body
Where in the cell is a TH receptor found?
In the nucleus
What is the cellular response to TH receptor activation?
Receptors will dimerize and form a DNA binding protein to regulate transcription
What is the primary effect of thyroid hormone? How is this accomplished?
Increase cellular metabolism
- Increase Na/K -ATPase activity
- Increase synthesis of respiratory enzymes
- Increase substrate availability
- Increase cellular heat production
- Effects on mitochondria
What are some secondary effects of TH?
- Activates sympathetic nervous system
- Increases sensitivities to catecholamines
- Nervous system development
- Regulates growth hormone
T/F: Thyroid disease is more likely in men.
FALSE
More likely in women
What is the most likely cause of thyroid disease?
Disruption of feedback mechanism causes over or under production of TH
T/F: Hypothyroidism is much more common than hyperthyroidism.
TRUE
What are the leading two causes of hypothyroidism?
- Iodine deficiency
2. Damage to gland
T/F: Only hypothyroidism can cause a goiter.
FALSE
They both can
During an iodine deficiency, why would a goiter form?
Not enough TH secretion -> no negative feedback -> excess TRH and TSH causes goiter
How are the two major causes of hypothyroidism treated?
Iodine deficiency - iodized salt
Damage to gland - exogenous TH
T/F: Iodine deficiency during pregnancy can have extreme effects.
TRUE
Cretinism, miscarriage, stillbirth
What are some symptoms of hypothyroidism?
- Abnormal concentrations of TH and TSH
- Goiter
- Sensitivity to cold (mild)
- Slight weight gain (mild)
- Myxedema (severe)
What are some primary defects that can cause hyperthyroidism?
- Thyroid tumor
2. Inflammation of gland
What disease can be associated with hyperthyroidism?
Graves disease
In a patient with Graves disease, what would be the relative concentrations of TSH and TH in the blood?
In Graves disease antibodies are made that can activate TSH receptors stimulating TH production with no TSH
Therefore TSH (and TRH) concentrations are low, but TH concentrations are high
If a goiter forms even though TSH levels are low what should be suspected?
Graves disease: stimulation of TSH receptors via antibodies with little TSH
What is the treatment for hyperthyroidism?
Removal of gland or destruction with radioactive iodine
If a patient comes in with an odd amount of swelling around the lips and tongue what could be suspected?
Myxedema from hypothyroidism
T/F: Hypothyroidism is related to slowed tooth development.
TRUE
Less TH = slower development
From its point of secretion until it reaches its target cells, TRH is found in or passes through which structures?
Hypothalamus -> blood vessels -> anterior pituitary
What is the effect of thyroid hormone during infancy?
Affects growth and CNS maturation
Cretinism, cold intolerance, myxedema, and reduced appetite are all symptoms of __________.
Hypothyroidism
Rapid heart rate is a symptom of __________.
Hyperthyroidism
T/F: Increased T3 and T4 secretion will increase TRH secretion.
FALSE
Decrease via negative feedback
T/F: Hypothermia will cause an increase in TRH secretion.
True
Increases metabolic rate to warm up
What will be the result of an iodine deficiency relating to TSH, T3 and T4, TRH, and goiter?
- Increased TSH secretion
- Decreased T3, T4 production
- Increased TRH production
- Goiter development
