Thyroid function

Question 1

What is the normal level of free T3 in the blood?

Answer 1

3 - 9 pmol/L

Question 2

What is the normal level of free T4 in the blood?

Answer 2

10 - 25 pmol/L

Question 3

What is the normal level of TSH in the blood?

Answer 3

0.35 - 3.30 mU/L

Question 4

Describe two cell types present in the thyroid

Answer 4

C (clear) cells
 - secrete calcitonin (not a thyroid hormone)
Follicular cells
 - support thyroid hormone synthesis
 - surround hollow follicles
 - produce T3 and T4
 - produce thyroglobulin (contains tyrosine residues)
 - produce thyroid hormone enzymes

Question 5

Which cells produce thyroid hormones?

Answer 5

Follicular cells

Question 6

What fills the centres of the follicles?

Answer 6

Colloid (glycoprotein matrix)

Also contains thyroglobulin and stored thyroid hormones

Question 7

Describe how thyroid hormones are synthesised

Answer 7

Iodide enters the follicular cells from the plasma via a Na+/I- transporter (against conc grad). Iodide is then transported out of the follicular cell into the colloid via the pendrin transporter.
Thyroid peroxidase enzyme catalyses the addition of one or iodide to tyrosine residues in thyroglobulin.
This creates monoiodotyrosine and diiodotyrosine.
MIT an DIT are added together to make triiodothyronine (T3) or tetraiodothyronine (T4)
In response to TSH, portions of the colloid are endocytosed into the follicular cell.
Within the cells they form vesicles which contain proteolytic enzymes that cut the thyroglobulin to release thyroid hormones.

Question 8

Where is the thyroid peroxidase enzyme present?

Answer 8

On the colloidal side of the follicular cells

Question 9

What are the two possible names give to the T4 hormone?

Answer 9

Tetraiodothyronine

Thyroxine

Question 10

Which hormone inhibits release of TSH from the anterior pituitary?

Answer 10

Somatostatin (aka growth hormone–inhibiting hormone)

Question 11

Where in the cell are thyroid hormone receptors located?

Answer 11

Inside the target cells, around the nucleus (nuclear receptors)

Question 12

Describe the main functions of thyroid hormones

Answer 12

• raises metabolic rate and promotes thermogenesis (brown fat is very responsive to thyroid hormone, present in high quantities in babies to keep them warm).
• increase hepatic gluconeogenesis
• net increase in proteolysis
• net increase in lipolysis
• critical for growth (lack of TH results in retarded growth); stimulates GH receptor expression
• required for foetal brain development (deficiency = congenital hypothyroidism)

Question 13

Describe the cause and effects of congenital hypothyroidism

Answer 13

Deficientcy can be caused by dietary iodine deficiency in the mother
Brain retardation which cannot be overcome by supplementary treatment

Question 14

Give the main causes of hypothyroidism

Answer 14

Hashimoto's disease (autoimmune)
Iodine deficiency (diet)
Idiopathic

Question 15

Give the main causes of hyperthyroidism

Answer 15

Grave's disease (antibodies mimic TSH)
Thyroid adenoma (hormone-secreting thyroid tumour)