L5 Thyroid Gland Flashcards
Describe the structure of the thyroid gland.
- Overlies the trachea
- 2 large asymmetrical lobes connected by an isthmus
- 4 parathyroid glands found posteriorly (control plasma calcium)
- Rich blood supply
Which organs regulate the thyroid gland?
The hypothalamus and pituitary gland.
What is the overall function of thyroid hormones?
Controls metabolism, growth and development.
Describe the histology of the thyroid gland.
- Functional unit of gland = follicle
- Each follicle is composed of a layer of follicular cells surrounding a colloid filled cavity
- Separations made by follicular epithelium
What are the 2 main thyroid hormones?
- Triiodothyronine (T3)
- Thyroxine (T4)
Describe triiodothyronine.
T3:
- 5ug produced per day
- Much greater biological activity than T4
- Contains 3 atoms of iodide per molecule
Describe thyroxine.
T4:
- 80ug produced per day
- Contains 4 atoms of iodide per molecule
Both T3 and T4 are in a protein bound form in plasma, what are these proteins?
Carrier-proteins:
- Thyroxine-binding globulin
- Albumin
Describe thyroid hormone synthesis.
- Tyrosines provided by thyroglobulin are secreted into the lumen of follciles as colloid
- Iodine is pumped into the follicular cells and binds to the tyrosine
- Lyososomal enzymes remove the rest of the thyroglobulin molecule so only the iodinated tyrosine remians
What causes T4 or T3 to be produced?
Iodine is consumed in the diet and enters cells as diiodotyrosine or monoidotyrosine depending on how many iodines are bound.
2 diiodytorisne = T4
1 diiodotryosine + 1 monoidotyrosine = T3
Which thyroid hormone is functional?
Triiodothyronine (T3) is functional.
T4 must be converted to T3 in peripheral tissues e.g. liver and kidneys by enzymes
Describe the process of T3 binding to cells.
- Cells uptake T3
- There are nuclear receptors called thryoid responsive elements (TRE) which bind the hormone
- Causes transcription of specific genes
- Causes a cellular effect which leads to a whole body effect
What are the 4 principal actions of triiodothyronine?
- Metabolism: increased mobilisation and utilisation of glucose, fat and proteins
- Heat production: calorigenic effects to regulate temperature
- Growth and development (e.g. CNS development)
- Cardiovascular function: increases cardiac output, heart rate and contractility
Explain the hypothalamic-pituitary-thyroid axis (HPT).
- Hypothalamus produces thyrotropin releasing hormone (TRH)
- Stimualtes anterior pituitary to secrete thyroid stimulating hormone (TSH)
- TSH acts on the thyroid gland to produce T3 and T4
It is a negative feedback loop.
What are the causes of hypothyroidism?
- Iodine deficiency (aka endemic goitre): enlarged thyroid gland due to trophic effects of TSH, insufficient T3 and T4, abnormally high TSH.
Treated with iodine supplements and levothyroixne. - Hashimoto’s thyroiditis (autoimmune disease): body produces antibodies against TSH receptors in thyroid gland, prevents T3 and T4 production, causes thyroid gland destruction.
