Jan 15 - Thyroid Gland Flashcards
Describe the thyroid gland
It consists of two lobes of endocrine tissue joined in the middle by narrow portion of gland
What are the two types of cells that make up the thyroid gland?
Follicular cells
C cells
Describe follicular cells
They are arranged into hollow spheres
They form the functional unit called a follicle
They are responsible for the production of thyroid hormones thyroxine (T4) and triiodothyronine (T3)
The lumen is filled with colloid, which serves as extracellular storage site for thyroid hormones
What are the two names for the thyroid hormone T4?
Thyroxine or tetraiodothyronine
What is the difference between T3 and T4 (besides the number of iodine molecules)?
T4 is the major form in terms of production
T3 is metabolically active (T4 is a prohormone)
What is the role of C cells?
They secrete the peptide hormone calcitonin
What are the basic ingredients of thyroid hormones?
Tyrosine (synthesized in sufficient amounts by the body)
Iodine (obtained from dietary intake)
What does an iodine deficiency cause?
Goitre
Where does synthesis of the thyroid hormones take place?
All steps occur on the thyroglobulin molecules within the colloid
What are the steps of thyroid hormone synthesis?
Tyrosine-containing thyroglobulin is exported from follicular cells into colloid by exocytosis
Thyroid captures iodine from blood and transfers it into colloid by iodine pump.
Within colloid, iodine attaches to tyrosine. The attachment of one iodide to tyrosine yields monoiodotyrosine (MIT). The attachment of iodides to tyrosine yields diiodotyrosine (DIT).
Coupling process occurs between iodinated tyrosine molecules to form thyroid hormones. The coupling of MIT and DIT yields T3. The coupling of two DITs yields T4
Where are thyroid hormones stored?
Thyroid hormones remain in the colloid until they are split off and secreted. Usually there is enough thyroid hormones stored to supply a body’s need for several months (it takes several months before symptoms as a result of thyroid problems arise).
Describe the secretion of thyroid hormones
When the body needs thyroid hormones, the follicular cells phagocytize thyroglobulin-laden colloid. These pseudopods are integrated with lysozymes. This process frees T3 and T4 to diffuse across the plasma membrane and into the blood.
What are the effects of thyroid hormones?
They are the main determinant of basal metabolic rate. They influence synthesis and degradation of carbohydrate, fat and protein. They increase target-cell responsiveness to catecholamines. They increase heart rate and force of contraction. They are essential for normal growth. They play a crucial role in the normal development of nervous system
How is the hypothalamus-pituitary-thyroid axis regulated? Why is it important?
It is regulated by negative-feedback system between hypothalamic TRH, anterior pituitary TSH, and thyroid gland T3 and T4. The feedback loop maintains thyroid hormones relatively constant
What are the causes of hypothyroidism?
Primary failure of thyroid gland
Secondary to a deficiency of TRH, TSH or both
Inadequate dietary supply of iodine
Why is it important to determine what causes hypothyroidism?
You don’t want to give someone TSH if the problem is that the thyroid has stopped working
What are symptoms of hypothyroidism?
Sleepiness, weight gain, always cold, problems with memory, problems forming words
What is cretinism?
It results from hypothyroidism from birth. It’s quite severe. Babies are tested at birth to ensure they have sufficient thyroid hormones
What is myxedema?
It’s a term often used for severe hypothyroidism in adults. It frequently looks like someone has swollen hands and feet. Often their skin is coarse in appearance. Usually, replacement therapy with T4 is the solution (unless it’s dietary - iodine)
When the cause of hypothyroidism is primary failure of the thyroid gland, what are the plasma concentrations of relevant hormones? Is there a goitre present?
There’s a low concentration of T3 and T4, and high concentration of TSH. There is a goitre present because TSH will cause the follicular cells to proliferate
When the cause of hypothyroidism is secondary to hypothalamic or anterior pituitary failure, what are the plasma concentrations of relevant hormones? Is there a goitre present?
There is a low concentration of T3, T4, TSH and/or TRH. There is no goitre present
When the cause of hypothyroidism is lack of dietary iodine, what are the plasma concentrations of relevant hormones? Is there goitre present?
There is a low concentration of T3, T4 and a high concentration of TSH. There is a goitre present because TSH will cause the follicular cells to proliferate
What is the most common cause of hyperthyroidism? Describe it
Graves’ disease. It’s an autoimmune disease where the body erroneously produces thyroid-stimulating immunoglobins (TSI). Since TSI isn’t controlled by the negative-feedback loop, the thyroid is constantly being stimulated. It’s characterized by exopthalmos (the bulging of the eyes due to a build up of fluids behind the eye)
How is hyperthyroidism treated?
Surgical removal of a portion of the over-secreting thyroid
Administration of radioactive iodine
Use of antithyroid drugs
