Endocrinology 4 - Hypothyroid Disorders

Question 1

What are the thyroid hormones?

Answer 1

• Thyroxine (tetraiodothyronine) is a prohormone converted to active T3 by deiodinase
• T3 (triiodothyronine) 20% from secretion, 80% from deiodination of T4, binds to thyroid response element

Question 2

What is the usual drug of choice in thyroid hormone replacement therapy?

Answer 2

• Levothyroxine sodium (thyroxine/thyroxine sodium/T4)

- Liothyronine sodium (T3 - less commonly used)

Question 3

List the clinical uses of levothyroxine sodium

Answer 3

• Treatment of primary hypothyroidism (oral administration, TSH used as guidance)
• Secondary hypothyroidism (oral admin, TSH is low so cant be used to guide dose, T4 is used instead)
• Treat myxodema coma (IV T3 then oral when possible)

Question 4

List the causes of primary hypothyroidism (myxoedema)

Answer 4

• Autoimmune
• Iatrogenic
• Radioiodine treatment

Question 5

List the causes of secondary hypothyroidism

Answer 5

• Pituitary tumour
• Post surgery
• Radiotherapy

Question 6

Compare the positives and negatives of combined thyroid hormone replacement

Answer 6

• Improved well-being has been reported - but no evidence that its better for you
• Symptoms of toxicity - palpitations, tremor, anxiety (biologically potent)

Question 7

Describe the pharmacokinetics of thyroxine replacement drugs

Answer 7

• Active orally
• Long half life (levothyroxine (T4) 6 days, liothyronine (T3) 2.5 days)
• 99.97% T4 bound to plasma proteins (TBG), and 99.7% T3. This will be unavailable to tissues

Question 8

What affects plasma binding protein levels?

Answer 8

• Pregnancy increases plasma binding proteins, as does oestrogen and phenothiazides
• TBG falls in malnutrition and liver disease
• Some co-administered drugs (phenytoin and salicylates) compete for protein binding sites and displace bound thyroxine

Question 9

What happens to TSH in primary hypothyroidism?

Answer 9

TSH increases

Question 10

Draw the hypothalamo-pituitary thyroid axis

Answer 10

TRH, TSH, T3/T4

Question 11

List the symptoms of myxoedema (primary hypothyroidism)

Answer 11

• No thyroxine, high TRH and TSH
• Myxoedema coma
• Hypertension
• Slow pulse
• Ascites
• Weakness
• Sensation of coldness
• Voice changes (deepening)
• Hairloss (armpit, pubic, head hair, eyebrow)
• Depression
• Tiredness
• Weight gain
• Constipation

Question 12

What is thyroglobulin?

Answer 12

Cells in the colloid that contain iodine, T4 and T3

Question 13

Why does hypothyroidism result in heavy periods?

Answer 13

Without thyroid hormone, your ovaries don’t make enough progesterone