59 - Thyroid Function in Health and Disease Flashcards
Roles of thyroid gland
1
2
- Secrete thyroid hormones to regulate metabolism
* Secrete calcitonin to regulate calcium levels
Structure of thyroid gland
Two lobes, joined by an isthmus below larynx
Alternative name of T4
Thyroxine
Where in the thyroid gland are T3 and T4 made?
Colloid
Structure of the thyroid
1
2
3
• Follicular cells surrounding colloid
• Colloid (glycoprotein mix) contains thyroglobulin (Tg)
• C cells in interstitial spaces secrete calcitonin
– Calcium metabolism
Proportion of T3 derived from T4
80%
Are thyroid hormones lipophilic or hydrophilic?
Lipophilic
Majority of thyroid hormones produced by the thyroid
T4 (90%)
Most potent of the thyroid hormones
T3 (tri-iodotyrosine)
Receptor for thyroid hormones
Nuclear
Whole-body effects of THs
Increase in O2 consumption (thermogenesis)
Protein catabolism in adults.
Anabolism in children.
Normal development of the nervous system.
Action of THs at cellular level
Increases activity of metabolic enzymes and Na+/K+ ATPase
Only stimulus that increases TRH levels
Cold temperatures (only in infants)
Where do T3 and T4 negatively feedback?
Both on hypothalamus (inhibits tonic release of TRH) and anterior pituitary (TSH release)
Half life of THs
About 1 day for T3
6-7 days for T4
Binding proteins of THs in the blood
Thyroxine-binding globulin and albumins
Main actions of THs
1
2
3
• Increase BMR & oxygen consumption – Calorigenic / heat producing effects • Modulate metabolism • Sympathomimetic effect; Increase heart rate, cardiac output
Developmental roles of THs
1
2
3
• Not essential for life but essential for child
growth & development
• Growth promoting
– Fetal life
– After birth essential for GH & IGFs actions
• Nervous system development
– Before & after birth
Hormone levels in primary hypothyroidism and iodine deficiency
Decreased T3, T4, increased TSH
Most common endocrine illness
Hypothyroidism (1-2% of adults, more common in women than men)
Hormone levels in secondary hypothyroidism
Decreased T3, T4, decreased TSH, can have decreased TRH
Hypothyroidism without a goitre
Secondary hypothyroidism (EG: from hypothalamic or anterior pituitary failure/tumour)
Physiological effects of hypothyroidism 1 2 3 4 5
– Decrease BMR & O2 consumption, decrease energy, increase weight
– Cold intolerance
– Sympathetic effects, decrease heart rate
– Central effects, decrease mentation & reflexes, fatigue
– Decrease protein synthesis, accumulate mucopolysaccharides under skin
= puffy appearance of myxedema
Causes of congenital hypothyroidism
1-5
1) Maternal iodine deficiency
2) Foetal thyroid dysgenesis
3) Inborn errors of thyroid hormone synthesis
4) Maternal antithyroid antibodies cross placenta
5) Foetal hypopituitary hypothyroidism
Hormonal signs of primary hyperthyroidism
High T3 and T4, decreased TSH
Most common cause of primary hyperthyroidism
Grave’s disease (60-80% of cases)
Gender in which Grave’s disease is more common
Women
Hormonal signs of secondary hyperthyroidism, from hypothalamic or anterior pituitary excess
High T3 and T4, high TSH, +/- high or low TRH
Hormonal signs of secondary hyperthyroidism from a hypersecreting tumour of the thyroid
Increased T3 and T4, decreased TSH.
No goitre.
Physiological effects of hypethyroidism
1
2
3
– Increased BMR & O2 consumption, increased appetite, weight loss, muscle weakness, heat intolerance, increased heart rate
– Increased alertness, irritability, hyperexcitability
– Graves’ disease exophthalmos = bulging eyes
What leads to exophthalmos in Grave’s disease?
Water retaining carbohydrates behind eyes
Treatment of hyperthyroidism
1
2
3
– remove thyroid / tumour
– use radioactive iodine to destroy gland
– Anti thyroid drugs
• block TH synthesis or conversion T4 to T3
How does goitre occur in hypothyroidism?
Decreased T3 and T4 from low iodine.
No negative feedback onto anterior pituitary.
Ant pit releases TSH, hyperstimulating thyroid
How does a goitre occur in hyperthyroidism?
Thyroid stimulating immunoglobulins hyperexcite thyroid.
