2/15: Thyroid and Parathyroid Hormones Flashcards
What hormone is the most potent in the thyroid gland?
T3 is more potent thatn T4
About 93% of the active hormones secreted by the thyroid gland is _________, while 7% is __________
Thyroxine (T4)
Triiodyhronine (T3)
What do thyroid hormones impact?
metabolism
and growth/development. They also
have permission action on
catecholamines
What does calcitonin do?
Decrease plasma calcium
Wha tis required for thyroid hormone synthesis?
Iodine (I2) so thyroid follicular cells actively transport iodide (I-) obtained from the diet
How does iodide exit the thyrocyte?
Across the apical membrane
to access the colloid,
where the initial
steps of thyroid
hormone synthesis
occur
What is pendrin?
A Cl-/I- exchanger
What is the Na+/I- symporter (NIS) capable of producing?
intracellular I- concentrations that are 20–40 times as great
as the concentration in plasma
Where are T3 and T4 produced?
In the colloid
What are T3 and T4 complexed with?
Thyroglobulin (Tg)
What are the steps of T3 and T4 secretion into blood?
- Colloid is internalized by endocytosis.
- The vesicles fuse with lysosomes in the cell.
- Proteases cleave T3 and T4 from TG.
- T3 and T4 diffuse out of the cell and into
capillaries
99% of T3 and T4 bind with what plasma proteins for transport?
Thyroxine-Binding Globulin (TBG), Transthyretin (TTR), Albumin
What are T3 and T4 half lifes?
- Due to the strength of its binding to the transport protein, T4 has a long
half-life (6-7 days). - T3 doesn’t bind as tightly so its half-life is only 2-3 days
How do target cells make active T3?
By using enzymes called Deiodinases/iodinases that remove an iodine from T4
How do individual target cells alter their exposure to T3?
By regulating their tissue deiodinase synthesis
What conditions inhibit deiodinase activity?
selenium deficiency,
burns, trauma, advanced cancer, cirrhosis, chronic kidney disease,
MI and febrile states, fasting, stress. Could show signs of hypothyroidism
Do T4 or T3 actions occur sooner?
T3 - with the maximum activity 2-3 days
*slower onset, but higher duration of action
What is the synthesis of new proteins important for?
- other systems
- growth
- CNS development
- Cardiovascular
- metabolism
Where is there negative feedback when secreting thyroid hormones?
Mainly at the level of the anterior pituitary gland
What is the main circulating form of thyroid hormone?
T4 -> responsible for most of the (-) feedback
What are the effects of thyroid hormone on metabolism?
- Stimulates oxygen consumption by most metabolically active tissues.
- Increased Basal Metabolic Rate (BMR)
- stimulates carbohydrate metabolism
- stimulates protein catabolism and synthesis
- stimulates fat metabolism
How does thyroid hormone stimulate carbohydrate metabolism?
– Causes uptake of glucose by cells
– Enhances glycolysis & gluconeogenesis
– Increases rate of CHO absorption from G.I. tract
How does thyroid hormone stimulate fat metabolism?
– Increases lipid mobilization & oxidation of fatty acids by cells
– Required to convert beta carotene to vitamin A (Hypothyroid patients have
yellowish skin)
– Decreases circulating cholesterol levels (Hypothyroidism associated with hyperlipidemia)
What do thyroid hormones do for our nervous system?
- Needed for normal development of
the NS - Impacts reflex time (i.e.
hypothyroidism can cause prolonged
reflex times) - Muscle tremors due to increased
reactivity of neuronal synapses - Feeling of tiredness but difficulty
sleeping - Anxiety, worry and paranoia
What do thyroid hormones do for our cardiovascular system?
- Increased expression of β-
adrenergic receptors - Increased blood flow, heart rate, and heart contractility
What do thyroid hormones do for our endocrine system?
- Activation of bone
formation causes a need
for increased PTH
secretion
What do thyroid hormones do for our gastrointestinal system?
- Increased appetite and
food intake - Increased rate of
secretion and motility of
the GI tract (i.e.
hypothyroidism can
produce constipation)
What is goiter?
Enlarged thyroid that DOES NOT indicate functional status
- seen in Hypothyroidism, Hyperthyroidism, Euthyroidism
What is goiter caused by?
Excessive amounts of TSH secretion
– High TSH stimulates thyroid to secrete large amounts of thyroglobulin colloid into follicles, resulting in gland enlargement
What is the most common form of hyperthyroidism?
Grave’s disease
What is grave’s disease?
An autoimmune disease where antibodies to
TSH receptor called thyroid-stimulating immunoglobulins (TSIs) stimulate the thyroid
gland to excess
What do high levels of thyroid hormone secretion caused by TSI do?
Suppress anterior pituitary TSH secretion (negative feedback)
What are hyperthyroidism symptoms?
Fine hair
Exopthalmos
Nervousness
Goiter
Tachycardia
Increased appetite
Weight loss
Fine tremor
Pretibial myxedema
Oligomenorrhea
Muscle wasting
Sweating, heat intolerance
What is treatment of hyperthyroidism?
- Radioactive I 131 thyroid ablation, or
Antithyroid Drugs (propylthiouracil or
methimazole).
– Surgery rarely indicated. - Propanolol (b blocker) given for adrenergic symptoms while awaiting
resolution. - L-thyroxine administered to prevent hypothyroidism in patients who have undergone ablation or surgery
What are oral symptoms of hyperthyroidism?
- Burning Mouth Syndrome
- Gum disease
- Excessive salivation
- Weakening of mandible
- Increased caries risk
What is thyroid storm (thyrotoxicosis)?
- Elevated Thyroid Hormone with stressful events (trauma, surgery,
severe emotional distress) or serious illness (DKA, MI, etc.). - Symptoms: fever, tachycardia, elevated BP, nausea, vomiting,
diarrhea, breathing problems, etc
What does low calcium result in?
Neuronal hyper-excitability (tetany)
*carpal spasm due to HYPOcalcemia
