Thyroid Function in Health and Disease Flashcards
What are the functions of the thyroid gland?
Secrete thyroid hormones > regulate metabolism
Secrete calcitonin > regulate Ca levels
What is the structure of the thyroid gland?
2 lobes joined by isthmus
What is the location of the thyroid gland?
Below larynx
Above trachea
Anterior in neck
What cells surround the colloid?
Follicular cells/thyrocytes
Where is thyroglobulin located?
In colloid
Where are the C cells?
In interstitial spaces
What is the function of C cells?
Secrete calcitonin
How are thyroid hormones synthesised?
- Follicular cell synthesises enzymes and thyroglobulin for colloid
- Na/I symporter brings iodide into cell > pendrin transporter moves iodide into colloid
- Enzymes add iodine to tyrosine > make T3 and T4
- Thyroglobulin taken back into cell in vesicles
- Intracellular enzymes separate T3 and T4 from protein
- Free T3 and T4 enter circulation
What is the role of thyroglobulin?
Stops iodine from diffusing out
Where do we get our iodine from?
Diet
What are the uses of iodine in the body?
Thyroid hormone synthesis
What are the structural components of thyroid hormone?
Iodine
Tyrosine derivative
What are the proportions of T3 and T4 made by the thyroid gland?
T3 = 10% T4 = 90%
How much T3 is derived from T4?
80%
Why are thyroid hormones bound to plasma proteins?
Because lipophilic
Where is thyroid stimulating hormone (TSH) released from?
Anterior pituitary
What does TSH do?
Stimulates all steps in thyroid hormone synthesis
Where is thyroid releasing hormone (TRH) released from?
Hypothalamus
What increases levels of TRH secretion?
Cold in infants only known factor
What decreases levels of TRH secretion?
Stress
What is the half-life of T4?
6-7 days
What is the half-life of T3?
1 day
What are the factors affecting release of thyroid hormones?
Tonic release
What are the target tissues of thyroid hormones?
Most cells of body
What are the actions of thyroid hormones?
Increase basal metabolic rate and oxygen consumption Modulate metabolism Sympathomimetic effect - Increases HR - Increases CO
Is thyroid hormone essential for life?
No, but essential for child growth and development
How does thyroid hormone promote growth?
Foetal life
After birth, essential for GH and IGFs actions
When is thyroid hormone needed for nervous system development?
Before and after birth
When does goitre occur?
In both hypothyroidism and hyperthyroidism
What is primary hypothyroidism?
Thyroid failure
Decreased T3 and T4
Increased TSH
Do you get a goitre in primary hypothyroidism?
Yes, because of trophic effect of increased TSH
What happens when you have low dietary iodine?
Decreased T3 and T4
Increased TSH
Do you get a goitre in dietary iodine deficiency?
Yes, because of trophic effect of increased TSH
What causes secondary hypothyroidism?
Hypothalamic/anterior pituitary failure/tumour
What are hormone levels with secondary hypothyroidism when it’s a hypothalamic cause?
Decreased T3 and T4
Decreased TSH
Decreased TRH
What are hormone levels with secondary hypothyroidism when it’s an anterior pituitary cause?
Decreased T3 and T4
Decreased TSH
Increased TRH
Do you get a goitre with secondary hypothyroidism?
No, because no trophic effect of TSH
What causes cretinism?
Congenital deficiency of thyroid hormones
What is the most common endocrine illness?
Hypothyroidism
Who has hypothyroidism more: females or males?
Females
What is the most common cause of hypothyroidism?
Iodine deficiency
What are some causes of congenital hypothyroidism?
Maternal iodine deficiency
Foetal thyroid dysgenesis
Inborn errors of thyroid hormone synthesis
Maternal antithyroid Abs > cross placenta
Foetal hypopituitary hypothyroidism
How can congenital hypothyroidism be prevented?
Adequate iodine in diet during pregnancy
Are newborns tested for thyroid deficiency?
Yes
What are the physiological effects of hypothyroidism?
Decreased basal metabolic rate and oxygen consumption
Decreased energy
Increased weight
Cold intolerance
Decreased HR
Decreased mentation, reflexes, and fatigue
Decreased protein synthesis > accumulate mucopolysaccharides under skin = puffy appearance of myxedema
What is the treatment for hypothyroidism?
Replace thyroid hormone
Adequate dietary iodine
What are hormone levels in primary hyperthyroidism?
Increased T3 and T4
Decreased TSH
Decreased TRH
Do you have a goitre with primary hyperthyroidism?
Yes
What is Graves disease?
Autoimmune hyperthyroidism
Thyroid-stimulating Ig
- Targets TSH receptors
- Acts as TSH
Why do you get a goitre in Graves disease?
AutoAbs acting as TSH by stimulating TSH receptor drive gland to enlarge
What is the epidemiology of Graves disease?
60-80% of all cases of hyperthyroidism
More common in women
What are the hormone levels in secondary hyperthyroidism, when it’s caused by a hypothalamus excess?
Increased T3 and T4
Increased TSH
Increased TRH
What are the hormone levels in secondary hyperthyroidism, when it’s caused by an anterior pituitary excess?
Increased T3 and T4
Increased TSH
Decreased TRH
Do you get a goitre in secondary hyperthyroidism?
Yes, because of trophic effect of TSH
What are hormone levels in a hypersecreting tumour?
Increased T3 and T4
Decreased TSH
Do you get a goitre with a hypersecreting tumour?
No
What are the physiological effects of hyperthyroidism?
Increased - Basal metabolic rate - Oxygen consumption - Appetite - Weight loss - Muscle weakness - HR - Alertness Heat intolerance Irritability Hyperexcitability
What is Graves disease exophthalmos?
Bulging eyes because of water retaining carbohydrates behind eyes
What is the treatment for hypothyroidism?
Remove thyroid/tumour Use radioactive iodine > destroy gland Antithyroid drugs - Block thyroid hormone synthesis - Block conversion of T4 to T3
