Thyroid hormones Flashcards
What structure connects the two lobes of the thyroid gland?
Isthmus
What are the thyroid lobes made off?
- Lobules which are made up of spherical follicles
- follicles contain colloid which is surrounded by an outer layer of follicular cells (cuboidal epithelial)
What is colloid made up?
thyroglobulin which contains tyrosine
What is a c-cell
also called parafollicular cell, produces calcitonin
What are the two thyroid hormones?
Thyroxine (T4)
Triiodothyronine (T3)
How are the thyroid hormones synthesised?
Tyrosine + iodine > Monoiodotyrosine (MIT)
MIT + iodine > Diiodotyrosine (DIT)
DIT + DIT > Thyroxine (T3)
DIT+ MIT > Triiodothyronine (T4)
Where can iodine be found in diet?
- Seafood
- Eggs
- Cranberries
- Sea vegetables
- Iodized salt
By which process does iodide get into the follicle lumen ready to be used in synthesise of thyroid hormone?
- Iodide transported into follicle cell via active transport using energy generated by Na/K ATPase pump
- Then transported into lumen of follicle using pendrin protein
- Then oxidised back into iodine using hydrogen peroxide
What hormone is needed to attach iodine onto tyrosine?
Thyroid peroxidase
Which structure holds the MIT, DIT, T3 and T4 molecules during storage and synthesise?
Thyroglobulin
How are T3 and T4 moved from storage in colloid to the blood?
- The compound of thyroglobulin bound to T3 and T4 exocytosis from colloid into follicle cell where lysosomes breakdown
- Any MIT, DIT and thyroglobulin recycled
- T3 and T4 released from cell and travel to blood stream
How is radioactive iodine used?
Treatment: for hyper secretion of thyroid hormone radioactivity causes thyroid cells to die
Diagnosis: Thyroid only cells which take up iodine so used to trace if any thyroid glands left if meant to be surgically removed
What enzyme converts T4 to T3?
primarily Type 1 deiodinase but also Type 2 and 3
Why does T4 need to be converted to T3?
T3 has higher affinity for nuclear receptors (majority of thyroid receptors)
What three proteins are used to transport thyroid hormone?
- Thyroid binding globulin (majority T4)
- Thyroid binding pre-albumin
- Thyroid binding albumin (majority T3)
Where are the majority of thyroid hormone receptors?
- nucleus
- found in most cells in the body
What are the intracellular effects of thyroid hormones?
bind to nuclear receptors and cause:
- translocation, activation or suppression of genes
What are the metabolic actions of thyroid hormone?
- Inc O2 consumption
- Inc BMR
- calorigenic effect
- Carbohydrate metabolism
- Lipid metabolism
- Protein metabolism
In which ways are carbohydrates metabolism affected by thyroid hormone?
- inc absorption of glucose from GIT
- inc glycogenolysis
- inc gluconeogenesis
In what ways are lipid metabolism affected by thyroid hormone?
inc lipolysis which increases Free fatty acids, accelerating FFA oxidation and decreased cholesterol except LDL cholesterol which is taken up more
In what ways are protein metabolism affected by thyroid hormone?
Increase protein synthesis unless in excess in which case breakdown protein
What are the affects of thyroid in Heart?
- inc HR, CO, systolic BP
- dec diastolic BP
- Inc BF to skin
What are the affects of thyroid in Lungs ?
- inc ventilation rate
What are the affects of thyroid in GIT?
- inc appetite
- inc secretion of digestive jucies
- inc GIT motility (symptom = diarrhoea)
What are the affects of thyroid in reproductive?
- essential for normal reproduction and lactation
What are the affects of thyroid in MSK?
- promotes normal body growth
- normal maturation skeleton
- normal development of muscles
What are the affects of thyroid in Nervous system?
- promotes neuronal development in foetus and infant
- normal neuronal function in adults
- enhances effects of sympathetic nervous system, up regulates B1 receptors in heart
How are thyroid hormones regulated?
stress stimulates dec
cold stimulates inc
1) hypothalamus secretes Thyroid regulating hormone
2) Anterior pituitary releases Thyroid stimulating hormone
3) Thyroid gland releases T3 and T4
T3 and T4 have long loops neg feedback
What is TSHs action on thyroid gland?
- inc iodide trapping and binding by increasing gene transcription of co-transporter protein
- promotes thyroglobulin synthesis and secretion
- synthesises TPO
- stimulates T3 and 4 synthesis
- promotes colloid endocytosis into follicular cells
- inc size and no of follicular cells
- inc BF to thyroid
How are disorders of thyroid classified?
Primary, secondary and tertiary (same as all hormone disorders)
What are the causes of primary hypothyroidism?
(failure of thyroid gland to respond to TSH)
- thyroiditis
- iodine deficiency
- deficiency of other synthesis enzymes
- removal of thyroid
What is secondary hypothyroidism and what’s the cause?
deficient TSH production
cause: thyroid atrophy
What is tertiary hypothyroidism?
deficient TSH secretion due to defect in TRH
What are the developmental consequences of hypothyroidism?
Adults will have very short stature, impaired brain development, bad coordination and unsteady gait
What is congenital hypothyroidism?
Thyroid doesn’t fully develop and so is non-functional
baby will struggle to feed and be sleep and constipated
treated by giving thyroid hormone replacement
What are the three causes of hyperthyroidism?
1) autoimmune disease e.g. Graves
2) Thyroid adenoma
3) TSH-secreting adenoma (rare)
What is the pathology of graves disease?
- Antibody producing cells produce thyroid stimulating antibodies
- Thyroid produces excess T3 and T4
- results in very low TSH and TRH as lots of negative feedback
signs of graves disease?
Exophthalamos (bulging eyes)
Goitre (enlarged neck)
What are the more common symptoms or signs of hyperthyroidism?
Weight loss Palpitation Diarrhoea Fatigue Hyperactivity and irritability sweating and heat intolerance tachycardia Tremor Goiter Moist warm skin muscle weakness
What are the more common symptoms or signs of hypothyroidism?
mental sluggishness weight gain constipation fatigue cold intolerance Dyspnea paraesthesia bradycardia goiter dry skin carpal tunnel syndrome peripheral edema
