Thyroid and parathyroid gland
Tuesday, 16 November 2021 9:57 PM
Gross anatomy of thyroid
- Largest endocrine organ in the body
- Two lateral lobes joined by thin isthmus on either side of the
trachea
- Supplied by superior thyroid artery and inferior thyroid
artery
- Venous blood drains via superior / inferior thyroid veins
- Thyroid follicles are the structural and functional units of the
gland
○ Each follicle surrounded by mainly cuboidal epithelium
○ Filled with colloid
§ Mainly composed of thyroglobulin
□ Contains ~120 tyrosine residues
Thyroid hormones
- Two biologically active thyroid hormones
○ Tetraiodothyronine (T4 usually called thyroxine)
○ Triiodothyronine (T3)
- Derived from the iodination of tyrosine
- Difference between the two hormones
○ Thyroid secretes about 80mg of T4 but only 5mg of T3
per day
○ T3 has much greater biological activity , about 4X than
T4
○ 24mg/day of T3 is produced by peripheral
monodeiodination of T4
Biosynthesis of thyroid hormones
1. Iodide trapping
a. Iodine is taken up actively by NA-I symporter
i. NIS
2. Oxidation
a. Iodine is oxidized by thyroidal peroxidase to iodine
3. Iodination
a. Tyrosine residue in thyroglobulin is iodinated and
forms MIT and DIT
i. Monoiodotyrosine and diiodotyrosine
4. Coupling
, a. Iodine is oxidized by thyroidal peroxidase to iodine
3. Iodination
a. Tyrosine residue in thyroglobulin is iodinated and
forms MIT and DIT
i. Monoiodotyrosine and diiodotyrosine
4. Coupling
a. Iodothyronines are coupled together to form T3/4
i. MIT + DIT = T3
ii. DIT + DIT = T4
5. Endocytosis
a. When thyroid gland is stimulated, vigorous
endocytosis of colloid occurs
6. Fusion
a. Endocytic vesicles fuse with lysosomes inside the
follicular cell
7. Release
a. T3/4 are hydrolyzed from the thyroglobulin and
released into the circulation
Thyroid hormone transport
- They are not very soluble in water
○ But are lipid soluble
- They are found in circulation associated with binding
proteins
○ Thyroid hormone binding globulins
§ 70% of hormone
○ Pre-albumin
§ Transthyretin
□ 15%
§ Albumin
□ 15%
- Less than 1% is found free in circulation
- Free and albumin bound thyroid hormone is biologically
available to tissues
Activation and inactivation of TH
- T3 has much greater biological activity than T4
- 25% of T4 converted to T3 in peripheral tissues
○ E.g. liver and skeletal muscles
- In addition to T3, an equal amount of Reverse T3 may also
be formed
○ Has no biological activity
- Deiodination reactions in the peripheral tissues activate or
inactivate TH
○ Deiodinase type 1 (D1) located in the liver / kidney
and thyroid gland
○ Deiodinase type 2 (D2) located in skeleton muscles,
Tuesday, 16 November 2021 9:57 PM
Gross anatomy of thyroid
- Largest endocrine organ in the body
- Two lateral lobes joined by thin isthmus on either side of the
trachea
- Supplied by superior thyroid artery and inferior thyroid
artery
- Venous blood drains via superior / inferior thyroid veins
- Thyroid follicles are the structural and functional units of the
gland
○ Each follicle surrounded by mainly cuboidal epithelium
○ Filled with colloid
§ Mainly composed of thyroglobulin
□ Contains ~120 tyrosine residues
Thyroid hormones
- Two biologically active thyroid hormones
○ Tetraiodothyronine (T4 usually called thyroxine)
○ Triiodothyronine (T3)
- Derived from the iodination of tyrosine
- Difference between the two hormones
○ Thyroid secretes about 80mg of T4 but only 5mg of T3
per day
○ T3 has much greater biological activity , about 4X than
T4
○ 24mg/day of T3 is produced by peripheral
monodeiodination of T4
Biosynthesis of thyroid hormones
1. Iodide trapping
a. Iodine is taken up actively by NA-I symporter
i. NIS
2. Oxidation
a. Iodine is oxidized by thyroidal peroxidase to iodine
3. Iodination
a. Tyrosine residue in thyroglobulin is iodinated and
forms MIT and DIT
i. Monoiodotyrosine and diiodotyrosine
4. Coupling
, a. Iodine is oxidized by thyroidal peroxidase to iodine
3. Iodination
a. Tyrosine residue in thyroglobulin is iodinated and
forms MIT and DIT
i. Monoiodotyrosine and diiodotyrosine
4. Coupling
a. Iodothyronines are coupled together to form T3/4
i. MIT + DIT = T3
ii. DIT + DIT = T4
5. Endocytosis
a. When thyroid gland is stimulated, vigorous
endocytosis of colloid occurs
6. Fusion
a. Endocytic vesicles fuse with lysosomes inside the
follicular cell
7. Release
a. T3/4 are hydrolyzed from the thyroglobulin and
released into the circulation
Thyroid hormone transport
- They are not very soluble in water
○ But are lipid soluble
- They are found in circulation associated with binding
proteins
○ Thyroid hormone binding globulins
§ 70% of hormone
○ Pre-albumin
§ Transthyretin
□ 15%
§ Albumin
□ 15%
- Less than 1% is found free in circulation
- Free and albumin bound thyroid hormone is biologically
available to tissues
Activation and inactivation of TH
- T3 has much greater biological activity than T4
- 25% of T4 converted to T3 in peripheral tissues
○ E.g. liver and skeletal muscles
- In addition to T3, an equal amount of Reverse T3 may also
be formed
○ Has no biological activity
- Deiodination reactions in the peripheral tissues activate or
inactivate TH
○ Deiodinase type 1 (D1) located in the liver / kidney
and thyroid gland
○ Deiodinase type 2 (D2) located in skeleton muscles,
