The Thyroid Gland and Iodothyronines Flashcards
Describe the structure of the thyroid gland.
It has two lobes joined by a narrowing called the ISTHMUS
It may have a remnant of the thyroglossal duct called the PYRAMID
Describe the cellular arrangement of the thyroid gland.
The cells are arranged in follicles
There are FOLLICULAR CELLS around the outside with a proteinaceous substance called COLLOID inside the follicles
There are parafollicular cells between follicles
What three main things does TSH stimulate in the follicular cell?
Stimulates IODIDE CHANNELS to transport iodide from the blood into the cell at the basolateral membrane (facing the blood)
Stimulates synthesis of THYROGLOBULIN
Stimulates THYROID PEROXIDASE
Describe the formation of T3 and T4 in the follicular cell.
In the presence of hydrogen peroxide, thyroid peroxidase can convert the iodide (which has been pumped into the colloid by the pendrin pumps) into a reactive iodine form.
TSH stimulates synthesis of a protein called thyroglobulin (moves to the apical surface - facing colloid) which is a long array of amino acids containing TYROSYL RESIDUES.
The tyrosyl residues can be iodinated by the reactive iodine in ONE or TWO positions forming MONOIODOTYROSINE (MIT) and DIIODOTYROSINE (DIT) respectively.
Continued stimulation of thyroid peroxidase will cause coupling reactions forming T3 and T4
At which positions are tyrosine iodinated?
3 and 5
Describe how iodothyronines are transported in the blood.
To prevent uptake by non-target tissues, T3 and T4 are mainly transported by PLASMA PROTEINS
The majority of T3 and T4 are transported by THYROXINE BINDING GLOBULIN (TBG)
Some T3 and T4 is transported by ALBUMIN (binds more loosely)
The BIOACTIVE component is the T3 and T4 that is UNBOUND
How do the latent periods and half-lives of T3 and T4 differ?
Latent Periods: T3 = 12 hours T4 = 72 hours Half Lives: T3 = 2 days T4 = 7-9 days
Describe the deiodination of T4.
T4 can be deiodinated to produce T3 which is MORE ACTIVE
T4 can be deiodinated in a different position to produce REVERSE T3 (which is INACTIVE)
What are the main actions of iodothyronines?
Increase basal metabolic rate
Increase carbohydrate, fat and protein metabolism (in both directions: anabolic and catabolic)
Enhance the effects of catecholamines
Interact with other endocrine systems
Have effect on CNS
Increase vitamin C synthesis from retinal
Iodothyronines are important in growth and development. What disease is caused by a lack of iodothyronines?
Cretinism
Describe the action of iodothyronines on target cells.
Thyroid hormones are NOT steroid hormones but they are LIPID SOLUBLE
They bind to intracellular receptors and have a genomic effect
State two hormones that increase iodothyronine production.
Thyroid Stimulating Hormone (adenohypophysis)
Oestrogens
State some factors that decrease iodothyronine production.
T3/T4 negative feedback (direct and indirect)
Somatostatin
Glucocorticoids
Ingestion of large amounts of inorganic iodide
Thyrotrophin (autonegative feedback loop)
