Thyroid Anatomy and Physiology Flashcards
What does the thyroid gland consist of
Left and right lobes connected by a midline isthmus
Where does the isthmus lie
Below the cricoid cartilage
What covers the thyroid gland
Strap muscles of the neck and overlapped by the sternocleidomastoids
What encloses the thyroid gland and attaches it to the larynx and the trachea ?
The pre tracheal fascia
What accounts for the upward movement of the thyroid gland on swallowing
The pre tracheal fascia attaching it to the larynx
Where does the thyroid gland develop from
The floor of the pharynx in the position of the foramen caecum of the adult tongue
Down which structure does the thyroid gland move to during development
Thyroglossal duct
What is the thyroid gland composed of
Epithelial spheres called follicles, whose lumens are filled with a proteinaceous colloid containing thyroglobulin
What are the two basic cell types present in the follicles of the thyroid and what do they each secrete
Follicular cells - secrete thyroxine (T4) and triiodothyronine (T3)
Parafollicular or C cells - secrete calcitonin
What sort of things do thyroid hormones regular
Organogenesis, growth and development Energy expenditure protein, carbohydrate and fat metabolism gut motility bone turnover heart rate and contractility and peripheral vascular resistance beta-adrenergic receptor expression muscle contraction and relaxation menstrual cycle erythropoiesis (RBC production)
What is essential for normal thyroid function and how do we obtain it
Iodine
Obtained by ingestion of food such as seafood, seaweed, kelp, dairy, some votable and iodised salt
What is the recommended iodine intake for adults
150ug / day
Where is thyroglobulin synthesised
In the RER and is transported into the follicular lumen by exocytosis
How is iodine transported into the thyroid follicular cells
Via a sodium-iodide symporter on the basolateral membrane of the follicular cells
What is the enzyme that catalyses the process of oxidation of iodide to iodine and its binding (organification) to the tyrosine residues of thyroglobulin to form MIT and DIT
Thyroid peroxidase enzyme
What are DIT and MIT molecules linked by
TPO to form thyroxine and triiodothyronine in a process known as coupling
What are uncoupled MIT and DIT
Deoiodinated, the free tyrosine and iodide are recycled
What does the thyroid gland store
T4 and T3 incorporated in thyroglobulin and can therefore secrete T4 and T3 more quickly than if they had to be synthesised
What structure produces T4
Entirely the thyroid gland
What might changes in T3 concentration indicate
A change in th rate of peripheral conversion
May not be an accurate measure of the change in tyroid hormone production
What are the majority of circulating T4 and T3 bound to
Plasma proteins
Only the bound thyroid hormone is available to the tissues. True or False
False - only the unbound form is available to the tissues
What are some things that could cause an increase in thyroid binding globulin levels
Hereditary TBG excess (X-linked) pregnancy drugs - oestrogen, tamoxifen, opiates, Hepatitis Acute intermittent porphyria
What are some things that could cause a decrease in thyroid binding globulin levels
Genetically determined malnutrition chronic liver disease nephrotic syndrome drugs - androgens, corticosteroids, Cushing's syndrome Acromegaly
What stimulates the synthesis and secretion of T3 and T4
Thyroid-stimualting hormone (TSH)
Where is TSH released from
Anterior pituitary gland
What increases the release and production of TSH
hypothalamic thyrotrophin-releasing hormone
What is TSH
a glycoprotein secreted by the thyrotroph cells of the anterior pituitary
Composed of alpha and beta subunits that are non-covalently bound
Alpha subunit of TSH is unique to TSH. true or false
False - it is the beta that is unique to TSH
Alpha is the same as that of LH, FSH and human chorionic gonadotrophin
What are some causes of increased TSH secretion
Primary hypothyroidism Secondary hyperthyroidism Thyroid hormone resistance Primary adrenal insufficiency Dopamine antagonists amiodarone
What are some causes of decreased TSH secretion
Primary hyperthyroidism
Secondary hypothyroidism
Dopamine agonists
steroids
How do thyroid hormones enter cells
Via active membrane transporter proteins
What is the majority of T3 produced by
5’ deiodination of T4 in peripheral extra-thyroidal tissues (liver and kidney)
What is easier to interpret - free or total thyroid hormone and why
Free
The level of bound hormone alters with changes in the levels of thyroid -binding proteins
