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17 Cards in this Set

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Describe the synthesis of thyroxine (T4) and tri-iodothyronine (T3) in the thyroid gland.
1. Uptake of plasma iodide (I-) by follicle cells (need I- in diet).
2. Iodine formation and iodination of tyrosine residues on thyroglobulin in the colloid.
3. Coupling of iodinated tyrosine residues to form T3 and T4.
4. Breakdown of thyroglobulin & secretion of T3 and T4.
Describe the actions of T3 and T4.
- metabolism of carbohydrate, fat and protein
- basal metabolic rate
- O2 consumption, heat production

- thyroid stimulating hormone (TSH) output from pituitary
- Important for normal growth & development e.g. of brain, skeleton
What are the properties of T3 & T4?
- Bind to thyroxine-binding globulin
- T3 is approx. 4x more potent than T4
- Half lives (de-iodinated in liver):
> T4 - 7 days
> T3 - 2 days
- Most circulating T3 produced by peripheral de-iodination of T4
Describe thyroid stimulating hormone (TSH).
- released from anterior pituitary
- stimulates I- intake, T3 & T4 synthesis and secretion
- release is inhibited by T3 and T4
- measurement provides initial screen for thyroid function
List some types of thyroid disease
Hyperthyroidism (overactivity) e.g. diffuse toxic goitre (Graves' disease), autoimmune; toxic nodular goitre

Hypothyroidism (underactivity) e.g. congenital; Hashimoto's autoimmune thyroiditis; prior treatment with radioactive iodine

Goitre (swelling):
- diffuse: activity can be normal, increased or decreased
- nodular (single or multinodular): may need to exclude cancer
What are the clinical signs of hypothyroidism in infants? What must be done?
Mental retardation
Poor appetite & growth, sluggish
Cool, dry, yellow skin, protruding tongue
Decreased heart rate & temperature

Must treat early to prevent irreversible changes
Screen at birth
What are the clinical signs of hypothyroidism in adults?
Coarse, sparse hair
Cold, dry, thickened skin
Drowsiness, reduced mental activity
Slow, low voice, partial deafness
Slow reflexes (e.g. ankle)
Poor appetite, constipation
Menstrual irregularities
Sensitive to cold, decreased heart rate and cardiac output
What are the uses of T3 and T4?
- Thyroxine (T4) used orally in hypothyroidism:
> max effect in 10 days
> duration 3 weeks

- Start with small does in elderly b/c risk of angina, arrhythmias or heart failure
- Can cause osteoporosis
Triiodothyronine (T3) useful for speed of onset, e.g. in myxoedema coma
What are the symptoms of hyperthyroidism?
Weight loss, but increased appetite
Anxiety, agitation, mood distrubances
Heat intolerance, sweating
Weakness, tremor
What are the signs of hyperthyroidism?
Increased heart rate
Lid retraction
Exophthalmos (corticosteroids may be of value)
Increase sympathetic activity, sweating
May get arrhythmias &/or heart failure, angina
What is the treatment of hyperthyroidism?
Anti-thyroid drugs

If no natural remission occurs may need:
- partial thyroidectomy or
- radioactive iodine

Surgery required if have thoracic inlet obstruction
Thioureas for hyperthyroidism
Inhibit: iodination of tyrosine on thyroglobulin & thereby T3 & T4 synthesis

Propylthiouracil (also reduces peripheral T3 production)

Orally active: full effect only after depletion of stores (3-4 weeks)
What are the adverse effects of thioureas?
Reduced granulocytes - reversible if stopped quickly: patients must report sore throats & fevers
Nausea, jaundice
Joint pains
Beta Blockers for hyperthyroidism
Reduce some of the symptoms of hyperthyroidism e.g.:
- tachycardia, arrhythmias
- angina
- tremor, agitation

Useful whilst waiting for effects of thioureas & 131I, for hyperthyroid crisis & to prepare for thyroidectomy
Iodide for hyperthyroidism
- Small amounts in diet necessary for thyroid function
- High doeses inhibit release of T3 & T4: useful for short term treatment of hyperthyroidism
- effects in 24 hours
- used for thyrotoxic crisis & preparation for thyroidectomy
- allergic reactions can occur
Describe 131I as a treatment.
- taken orally, half life 8 days
- accumulated by thyroid
- beta radiation is localised & destroys thyroid
- used for hyperthyroidism & cancer of thyroid (if cells still take up I-)
What are issues associated with 131I in treating hyperthyroidism.
- Difficult to get right dose
- Hypothyroidism may develop any time after 131I
- Contraindicated in pregnancy & childhood
- Advise patients not to reproduce for a few months after treatment (131I affets germ cells)