Thyroid function in health and disease Flashcards
What are the two major functions of the thyroid gland?
Secrete thyroid hormones to regulate metabolism
Secrete calcitonin to regulate calcium levels
Describe the histology of the thyroid gland?
Follicle = colloid surrounded by follicular cells
C cells in interstital spaces
Capillaries in interstital spaces
What is contained in the colloid?
Glycoprotein mix containing thyroglobulin
What do the C cells in the thyroid gland secrete?
Calcitonin
Where in the thyroid gland are thyroid hormones synhtesised?
Colloid (where thyroglobulin is)
Describe the process of thyroid hormone synthesis?
Where in the thyroid gland is thyrogobulin synthesised?
Where is it stored?
Synthesised in follicular cells
Stored in colloid
How is iodine transported into follicular cells and colloid for thyroid hormone synthesis?
Na-I symporter brings I and Na into follicular cell from blood
Pendrin transporter moves I into colloid from follicular cell
Describe what happens to iodine once it enters the colloid?
Iodine added to tyrosine > MIT
Iodine added to MIT > DIT
MIT added to DIT> T3
DIT added to DIT > T4
What happens to thyroglobulin after the thyroid hormones have been added to it?
Reabsorbed into follicular cells
How are the thyroid hormones released from thyroglobulin?
Enzymes in follicular cells separate T3 and T4 from thyroglobulin
How do T3 and T4 enter the circulation?
Lipophilic > diffuse from follicular cells
How are T3 and T4 carried in the blood?
Why?
Thyroid binding globulin
Albumin
Transthyretin
Because they are lipophilic
Describe the structure of thyroid hormones?
T4: tetraiodothyronine
T3: triodothyronine
Where is most T3 derived from?
80% from T4: deiodinase
Describe the thyroid hormone control pathway?
Describe the half life of the thyroid hormones?
T4: 6-7 days
T3: 1 day
Describe the factors that affect the release of thyroid hormones?
Tonic release
Where is the receptor for thyroid hormones located?
Nuclear receptor on most cells and tissues
Describe the actions of thyroid hormones?
Increase BMR and oxygen consumption
Modulate metabolism
Sympathomimetic effect
Growth promoting
Nervous system development
Describe the hormone levels in primary hypothyroidism?
Is a goiter present? Why?
Low T3 and T4, high TSH (no negative feedback)
Goiter present , as high TSh stimulates growth of thyroid
Describe the hormone levels in secondary hypothyroidism?
Is a goiter present? Why?
Low T3 and T4
Low TRH and/or TSH (hypothalamic or pituitary failure)
No goiter, as there is not any TSH present to drive growth
What is the most common cause of hypothyroidism?
Iodine deficiency
List the causes of congenital hypothyroidism?
Maternal iodine deficiency
Fetal thyroid dysgenesis
Inborn errors of thyroid hormone synthesis
Maternal antithyroid antibodies (cross placenta)
Fetal pituitary hypothyroidism
Describe the hormone levels in primary hyperthyroidism?
Is a goiter present? Why?
High T3 and T4
Low TSH (negative feedback from T3 and T4)
Goiter present, as stimulating auto-Ab encourgaes growth
Describe the causes of primary hyperthyroidism?
Autoimmune: Grave’s disease, thyroid stimulatin immunoglobulin
Describe the hormone levels in secondary hyperthyroidism?
Is a goiter present? Why?
Due to hypothalamic or pituitary excess: high T3 and T4, high TRH and/or TSH > goiter (TSH)
Due to hypersecreting tumour: high T3 and T4, low TSH (negative feedback) > low goiter (low TSH)
