Thyroid disease

Question 1

What do secretion units of the thyroid gland contain?

Answer 1

They contain follicles that are lined by follicular cells and filled with colloid, the colloid contains thyroglobulin, which plays an important role in TH. Between the follicles are C-cells, which produce a
calcitonin

Question 2

What are the 3 main hormones of the thyroid gland?

Answer 2

T4 (thyroxine), T3 (tri-iodothyronine) and reverse T3, which is not biologically active but produced under certain activities

Question 3

Which is more metabolically active out of the thyroid hormones?

Answer 3

T3, about 4x more active

Question 4

What happens in cells that are sensitive to TH?

Answer 4

T4 is converted to T3 by deiodination, inside the cell T3 binds to hormone receptors, causing an increase in energy expenditure

Question 5

What are the half-lives of T4 and T3?

Answer 5

T4 - 9 days

T3 - 1.5 days

Question 6

What are the TH binding hormones? How much of T4 & T3 is bound?

Answer 6

More than 99.95% of T4 and T3 is protein bound, very small amounts of T4 are in pure form. There are 3 TH binding hormones

• thyroxine binding globulin - 70%
• albumin - 20%
• pre-albumin - 10%

Question 7

How are T3 and T4 produced in the thyroid gland? (long answer)

Answer 7

Of the iodide present in our diets, one-third is taken up by the thyroid gland via the blood, while the other two-thirds are excreted in the urine. The iodide is concentrated to about 20+ times that of plasma and oxidised into iodine by the follicular cells. The iodine can then be combined with tyrosine present in the thyroglobulin to form monoiodotyrosine and diiodotyrosine, which can then be coupled together to make T3 and T4 respectively. The hormones remained attached to thyroglobulin and stored in the colloid in this form.

Question 8

`What stimulates and promotes the synthesis of thyroid hormones?

Answer 8

TSH

Question 9

How is T3 and T4 secreted from the thyroid gland? (long answer)

Answer 9

Thyroglobulin is taken up by follicular cells during pinocytosis creating droplets. Lysosomes fuse with the colloid droplets and proteases digest the thyroglobulin to release T4, T3 and amino acids. The free T3 and T4 are quickly bound by TBG and are transported in the blood. Any remaining MIT and DIT are released and de-iodinated so that the iodine can be used again.

Question 10

What is another name for myxoedema?

Answer 10

Hypothyroidism

Question 11

What is another name for thyrotoxicosis?

Answer 11

Hyperthyroidism

Question 12

What is primary hypothyroidism?

Answer 12

A problem occurring in the thyroid gland

Question 13

What is secondary hypothyroidism?

Answer 13

Problem occurring in the hypothalamus or the anterior pituitary via the inadequate release of either TSH or TRH

Question 14

Who does hypothyroidism usually affect?

Answer 14

More prevalent in women between 50-60 years of age

Question 15

What is secondary hyperthyroidism?

Answer 15

A very rare disorder caused by a tumour in the anterior pituitary causing very high T4 and TSH

Question 16

What are T4 and T3 and TSH levels in hyper and hypothyroidism?

Answer 16

Total T4 tends to be high in hyper and low in hypo, total T3 is very high in hyper but can be normal in hypo, therefore T3 is a better indicator when testing for hyper and T4 is a better indicator when testing for hypo. Plasma TSH is high in hypo and low in hyper, used as a first line test in labs