Lecture 2: Thyroid Flashcards
Anatomy of thyroid gland
- biggest gland in the neck. It is situated in the anterior (front) neck below the skin and muscle layers.
- shape of a butterfly with the left and right thyroid lobes which wrap around the trachea.
- function of the thyroid is to make thyroid hormone. The function of the thyroid therefore is to regulate the body’s metabolism.
Function/Process of thyroid gland with T3 and T4
-The function of the thyroid gland is to take iodine, and convert it into tyrosine and then into thyroid hormones: thyroxine (T4) and triiodothyronine (T3) which are transported via bloodstream where they control metabolism (conversion of oxygen and calories to energy).
The normal thyroid gland produces about 80% T4 and about 20% T3, however, T3 possesses about four times the hormone “strength” as T4.
Examples of metabolic functions of thyroid glands
Energy metabolism Carbohydrate metabolism Protein metabolism Lipid Metabolism Cardiovascular- increases heart rate, output, systolic volume and contractility Beta adrenergic effects Bone growth GI tract motility Erythropoietin production
The 2 receptors related to thyroid hormone
1.The thyrotropin (TSH) receptor
TSH, acting through the TSH receptor, is the major stimulator of thyroid cell growth, differentiation and function.
2.Thyroid hormone receptors (TRs) exhibit a dual role as activators or repressors of gene transcription in response to thyroid hormone (T3).
Receptor isoforms and associated effects
The TRα and TRβ isoforms and TRα1 and TRβ1 isoforms.
Have genomic or non-genomic effects
Genomic signalling pathway directly influences genetranscriptionandtranslation
Non-genomic pathway involves more rapid, cellular changes, some of which also regulate gene expression through more indirect signalling.
What are the key players in thyroid hormone regulation hormonal axis
ie the organ and what it secretes and its associated function
- Hypothalamus:
Thyrotropin-releasing hormone (TRH)
control of thyroid stimulating hormone (TSH)
Pituitary: Thyroid stimulating hormone (TSH)
controls production of the thyroid hormones by binding toTSH receptorslocated on cells in the thyroid gland.
Thyroid: triiodothyronine (T3) and thyroxine (T4)
What are the major serum thyroid hormone binding proteins and purpose
Thyroxine-binding globulin [TBG or thyropexin],
binds thyroid hormones in circulation
Transthyretin [TTR or thyroxine-binding prealbumin (TBPA)]
a transport protein in the serum and cerebrospinal fluid that carries the thyroid hormone thyroxine (T4)
Albumin (HAS, human serum albumin)
Describe how thyroid hormone secretion is controlled and regulated
Draw a diagram
The main controller of thyroid hormone levels is the pituitary gland, a small gland the size of a peanut at the base of the brain.
When the level of thyroid hormones (T3 & T4) drops too low, the hypothalamus produces thyrotropin Releasing Hormone (TRH) which tells the pituitary gland to stimulate the thyroid gland (release TSH).
The pituitary gland produces Thyroid Stimulating Hormone (TSH) which stimulates the thyroid gland by binding to the TSH receptors to produce more hormones.
The pituitary senses this and responds by decreasing its TSH production.
What are the thyroid autoantibodies tested and what disease can they be a sign of
- Thyroid peroxidase antibodies (TPO).
These antibodies can be a sign of:
- Hashimoto disease (Hashimoto thyroiditis)
This is an autoimmune disease and the most common cause ofhypothyroidism. Hypothyroidism is a condition in which the thyroid doesn’t make enough thyroid hormones.
-Graves’ disease.
This is also an autoimmune disease and the most common cause ofhyperthyroidism. Hyperthyroidism is a condition in which the thyroid makes too much of certain thyroid hormones.
2.Thyroglobulin antibodies (Tg).
These antibodies can also be a sign of Hashimoto disease. Most people with Hashimoto disease have high levels of both Tg and TPO antibodies.
3.Thyroid-stimulating hormone (TSH) receptor.
These antibodies can be a sign of Grave’s disease.
Relationship between TSH and T3/T4 and explain
High T3 and T4 causes low TSH, this is because of the negative feedback loop where the levels of
T3 and T4 cause the pituitary gland to stop making TSH to lower the T3 and T4 levels.
Low T3 and T4 causes high TSH this is because of the positive feedback loop where the levels of
T3 and T4 cause the pituitary gland to start making lots of TSH to increase T3 and T4 levels
Laboratory Diagnostic tests (6) and their purpose
- Measurement of serum thyroid-stimulating hormone (TSH):
Measuring serum TSH is the best way to determine thyroid dysfunction.
Normal test results essentially rule out hyperthyroidism or hypothyroidism, except in hyperthyroidism secondary to a TSH-secreting pituitary adenoma or pituitary resistance to thyroid hormone and in some patients with central hypothyroidism due to disease in the hypothalamus and/or pituitary gland
2.Measurement of total serum T4 and T3: Immunometric assays measure total, both bound and free hormone, although almost all T4 is protein-bound.
3.Direct measurement of free T4 and T3:
Since free thyroid hormones are available to peripheral tissues, directly measuring serum free hormones avoids the pitfalls of interpreting total levels, which are influenced by the level of the binding proteins. Thus, serum free levels more accurately diagnose true thyroid function.
4.Measurement of thyroid autoantibodies:
Autoantibodies to thyroid peroxidase and, less commonly, to thyroglobulin are present in almost all patient’s thyroid autoimmune disease
Thyroid peroxidase autoantibodies are usually detected in patients with Graves’ disease.
5.Measurement of thyroglobulin:
The thyroid is the only source of this iodinated high molecular weight glycoprotein, which is readily detectable in normal patients and is usually elevated in patients with nontoxic and toxic goitre.
The principle use of serum thyroglobulin is mostly used as a tumour marker test to help guide thyroid cancer treatment.
6.Testing for radioactive iodine uptake:
This test has disadvantages in cost, time, and patient inconvenience. The isotope of choice is 123I, which exposes the patient to vanishingly small radiation. It is valuable in the differential diagnosis of hyperthyroidism
Define Hypothyroidism
characterised by unusually low hormone production due to underactive glands that cannot produce enough hormones to regulate your metabolism.
3 types of Hypothyroidism and description
Primary hypothyroidism– when your thyroid gland becomes diseased and cannot produce sufficient hormones
Subclinical hypothyroidism- early and mild form of hypothyroidism
Secondary hypothyroidism– when your pituitary gland isn’t stimulating your thyroid to produce enough hormones.
Causes of primary hypothyroidism (hashimoto)
Hashimoto’s thyroiditis-most common
Idiopathic hypothyroidism-
Irradiation of thyroid
Surgical removal
Late stage invasive fibrous thyroiditis
Iodine deficiency
Drug therapy (Lithium, Interferon)
Infiltrative Diseases:
Sarcoidosis, Amyloidosis
Scleroderma, Haemochromatosis
Signs and symptoms of hypothyroidism CHIMPFWD
Fatigue. Increased sensitivity to cold. Constipation. Dry skin. Weight gain. Puffy face. Hoarseness. Muscleweakness.
