Lecture 22: Thyroid disorders Flashcards
Where is the thyroid gland located?
Between the larynx and the trachea
What is the thyroid follicle?
The basic functional unit of the thyroid gland
What are the steps in the synthesis of thyroid hormones
What is the active thyroid hormone?
T3
Where is most T3 derived from?
T4 by deiodination
How are T3 and T4 found in the plasma?
Extensively protein bound
What are the thyroid hormones bound to?
Thyroxine-binding globulin (TBG) and transthyretin (TTR) thyroxine-binding prealbumin
How are thyroid hormones secreted?
- The Hypothalamus secretes thyrotrophin releasing
- TRH acts on the anterior pituitary to secrete thyroid stimulating hormone
- TSH acts on the thyroid hormone to secrete thyroid hormone
What is the euthyroid state?
Thyroid hormone secretion is normal
What is the Hypothyroid state?
Thyroid hormone secretion is subnormal (also referred to as myxoedema)
What is the hyperthyroid state?
Thyroid hormone secretion is excessive (also referred to as thyrotoxicosis)
What are the actions of the TSH?
- TSH does more than just stimulate T3 and T4 production
- TSH increases protein synthesis in follicular epithelial cells, increases DNA replication, and cell division
- Increases rough endoplasmic reticulum and cell machinery required for protein synthesis
- Thyroid will undergo hypertrophy if exposed to greater TSH concentration
- Results in goitre
How much of the circulatng thyroid hormone is T3?
20%
How much of the circulatng thyroid hormone is T4?
80%
Are thyroid hormones lipohillic or hydrophillic?
Lipophillic and easily cross cell membranes
How does T3 act?
Acts by inducing gene transcription in protein synthesis
What is the primary action of thyroid hormones?
Increase the bodies metabolic rate - rate of oxygen consumption and energy expenditure at rest
What are the effects of thyroid hormones on tissues?
- Increase basal metabolic rate
- Increase heat production
- Increase responsiveness to sympathetic input
- Permits normal growth and development
What are the effects of thyroid hormone on the nervous system?
- Permits normal growth and development
- Permits maintenance of normal activity
What is iodine deficiency disease?
Cretinism is the most extreme version. It causes mental retardation, reduction in physical growth, deaf mutism.
What causes cretinism?
- An inadequate dietary iodine intake
- Meternal iodine deficiency during pregancnt
What is now fortified with iodine?
Salt
What is the required iodine intake?
150mcg / day
What are the thyroid function tests?
- TSH
- Free t4
- Free t3
What are the signs and symptoms of hypothyroidism?
- Cold intolerance
- Modest weight gain
- Bradycardia
- Tiredness for no apparent reason
- Constipation
- Forgetfulness and personality changes (depression)
- Pale, dry coarse skin
- Puffiness of face
What is primary hypothyroidism?
- T3 or T4 below normal
- Increased TSH
What causes most cases of hypothyroidism?
Damage or loss of thyroid tissue or due to inadquate iodine consumption
What causes primary hypothyroidism?
- Hashimotos thyroiditis
What is hashimotos thyroiditis?
An autoimmune diseasse - antibodies to thyroid peroxidase. Its twice as common in woman. T4 decreases and TSH increases. Can result in a goitre
What is the treatment for primary hypothyroidism?
Levothyroxine
What is levothyroxine?
A synthetic hormone identical to thyroxine T4
What are the adverse effects of levothyroxine?
- Hair loss during the first few months of treatment
- Headaches
- Sleep problems (insomnia)
- Nervousness
- Fever, hot flashes and/or sweating
- Pounding heart beat or fluttering in chest
- Appetite changes, weight changes
What is the treatment for primary hypothyroidism?
Liothyronine
What is liothyronine?
Synthetic form of T3
What are the risk of liothyronien?
Osteoporosis and arrythmia
What is the half life of levothyroxine?
- 7 days in euthyroid
- 14 days in hypothyroidism
What is the half life of liothyronine?
2 days in euthyroid
What is secondary hypothyroidism?
Its uncommon. The pituitary doesnt produce TSH or the hypothalamus dosesnt produce TRH. T3 and T4 and TSH are below normal
What is myxoedema coma?
It is the end result of untreated hypothyroidism. It is progressive weakness leading to a loss of consciousness. Extreme hypothermia
What are the signs and symptoms of myxoedema coma?
Areflexia, seizures and respiratory depression
What drugs affect throid function?
Corticosteroids, Lithium, Amiodarone, Cholestyramine
How do corticosteroids affect thyroid function?
Can decrease basal production of TRH and TSH, consequently decreasing thyroid hormone levels
How does lithium affect thyroid function?
Inhibits the release of thyroid hormones and interferes with their peripheral deiodination
How does Amiodarone affect thyroid function?
Contains iodine and can cause both hypothyroidism and hyperthyroidism
How does Cholestyramine affect thyroid function?
Reduces the absorption of thyroxine
what are the Signs and symptoms of Hyperthyroidism?
- Heat intolerance
- Palpitations
- Weight loss (despite increased appetite)
- Restlessness and nervousness
- Fatigue
- Increased sweating
- Frequent bowel movements
- Goitre may be present
What is hyperthyroidism?
Thyroid produces an excess of T4. Reduction in TSH due to negative feedback loop. T3 is usually elevated as well
What are the signs of graves disease?
Goiter and exopthalmos
What is the most common cause of hyperthyroidism?
Graves disease
What is graves disease?
An autoimmune disease caused by thyroid stimulating immunoglobulin. It activates TSH receptor on thyroid follicular cells and increases secretion of thyroid hormones.
What is the treatment for hyperthyroidism?
- Surgery
- Radioactive iodine
What are the antithyroid drugs?
- Thioamides eg carbimazole, propylthiouracil
How do antithyroid drugs work?
They are accumulated by the thyroid, inhibit thyroid peroxidase - the enzyme responsivle for iodiniation of tyrosine and prevent hormone synthesis. Propylthiouracil also inhibits peripheral deiodination. Its effect is slow in onset 4-6 weeks.
