59 - Thyroid Function in Health and Disease Flashcards by Jack Cuthbertson

Roles of thyroid gland
1
2

Secrete thyroid hormones to regulate metabolism

* Secrete calcitonin to regulate calcium levels

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Structure of thyroid gland

Two lobes, joined by an isthmus below larynx

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Alternative name of T4

Thyroxine

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Where in the thyroid gland are T3 and T4 made?

Colloid

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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

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Proportion of T3 derived from T4

80%

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Are thyroid hormones lipophilic or hydrophilic?

Lipophilic

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Majority of thyroid hormones produced by the thyroid

T4 (90%)

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Most potent of the thyroid hormones

T3 (tri-iodotyrosine)

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Receptor for thyroid hormones

Nuclear

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Whole-body effects of THs

Increase in O2 consumption (thermogenesis)
Protein catabolism in adults.
Anabolism in children.
Normal development of the nervous system.

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Action of THs at cellular level

Increases activity of metabolic enzymes and Na+/K+ ATPase

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Only stimulus that increases TRH levels

Cold temperatures (only in infants)

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Where do T3 and T4 negatively feedback?

Both on hypothalamus (inhibits tonic release of TRH) and anterior pituitary (TSH release)

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Half life of THs

About 1 day for T3

6-7 days for T4

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Binding proteins of THs in the blood

Thyroxine-binding globulin and albumins

Main actions of THs
1
2
3

• Increase BMR &amp; 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.