Antithyroid Drugs

  • By: Terri
  • Date: May 21, 2010
  • Time to read: 4 min.


Thyroid gland produces 2 iodinated hormones

  • T3 (triiodothyronine)
  • T4 (tetraiodothyronine)

They promote metabolism, growth & development. Both hormone need iodine, the iodine binds to thyrosine in the thyroglobulin (colloid) to produce T3/T4. This process is called organification.


  • Graves disease
    • Increase TSH receptor stimulation
      • Increase production of T3 & T4 by thyroid gland
  • Types of treatment for hyperthyroidism:
    • Drugs that decrease hormone synthesis
    • Selective radiation with Iodine-131.
    • Surgical removal of part/all of the thyroid gland (thyroidectomy)
    • Use Propanolol (B-blocker) for treating increaed sympathetic effect of thyroid hormones.
      • Useful for preoperation & thyroid storm


  • Usually caused by Autoimmune thyroiditis (Hashimoto)
    • Primary thyroid failure
  • Symptoms:
    • Reversible slowing of body functions
    • Lethargy
    • Tiredness
    • Decreased BMR
    • Decreased appetite
    • Decreased Cardiac output
    • Dry skin
    • Cold intolerance
    • Increased weight
    • Constipation
    • Myxoedema
  • Diagnosis
    • Low free thyroxin (T3,T4)
    • but High TSH (Primary hypothyroidism)
  • In infants, may lead to cretinism (dwarfism) & mental retardation
  • Requires thyroid replacement therapy (thyroxine)


Drugs used to treat hyperthyroidism

a. Drugs that decrease hormone synthesis

1. Thionamides (methimazole, propylthiouracil)


  • Inhibit organification by blocking enzyme thyroxine peroxidase which promotes coupling of iodide with tyrosine (to produce MIT/DIT)
  • Onset of effect up to 3-4 weeks
    • Long duration due to time for depletion of horrmone that were already produced, requires 3-4 weeks to be used up)
  • All given par-oral
  • Drug accumulates in thyroid
  • Goal is to administer thionamides until patient is euthyroid and then decrease maintanence dose to prevent lapse into hypothyroidism
  • May stop treatment after 1-2 years
  • Cross sensitivity between propylthiouracil & methimazole ~50%
    • Both sensitive by 50%

Adverse effects:

  • Immunosuppressive
  • Agranulocytosis
    • manifests commonly in sore throat
  • Other
    • Nausea
    • Taste disturbance
    • Pruritic rash
    • Fever
    • Vasculitis
    • Arthralgia (joint ache)
    • Cholestatic jaundice
    • hepatitis
    • lymphadenopathy
    • hypoprothrombinaemia

2. Propylthiouracil

  • Inhibits peripheral deiodination of T4 to T3
  • Reserved for patients who are intolerant to Carbimazole
    • 1/10 activity of Methimazole

Adverse effect: High Agranulocytosis

  • Fever/Sore throat should be reported (precedes agranulocytosis)

3. Carbimazole (prodrug)

  • Drug of choice in Grave’s disease*
  • Converted in vivo to methimazole
    • 1st pass effect

4. Anion Inhibitors (Potassium Perchlorate)

  • Inhibits the uptake of iodine (competitively, so organification cannot happen without iodine)

Adverse effect: Aplastic anaemia

  • Therefore, rarely used.

5. Iodide (Supersaturated potassium iodide solution SSKI, Lugol’s solution)

  • Inhibits organification & hormone release
  • *Decreases size & vascularity of gland
    • Useful for pre-thyroidectomy
    • Goiters are highly vascular (prevent excessive blood loss)
  • Reduces conversion of T4 to T3
  • Inhibits thyrotropin action

Course of action:

  • Quick effect (2-7 days)
    • Useful in thyroid storm
    • Faster than thionamides (3-4 weeks)
  • Short term treatment
    • antithyroid effect decreases

Adverse effects:

  • Iodism (rare)
    • Can be easily reversed by withdrawal of iodide
    • Seen in iodism:
      • Rash
      • Swollen salivary glands
      • Conjunctivitis
      • Rhinorhoea
      • drug fever
      • metalic taste

b. Selective radiation with Iodine-131

  • Emits beta rays which destroys thyroid tissue within few weeks
  • Given par-oral
  • Drug concentrated on thyroid (only act on thyroid)


  • Stabilize patient with Carbimazole 1st
  • Stop Carbimazole 3-4 days before administer Iodine-131
    • Takes 3-4 days to become euthyroid


Drugs used to treat hypothyroidism

a. Thyroxine – T4 (Levothyroxine sodium)

  • Treatment of choice**
  • Na+ salt of L-thyroxine
    • levothyroxine sodium
    • Synthetic (most widely used thyroid hormone)
  • Given par-oral

Course of action:

  • Effects delayed until plasma protein & tissue binding sites are occupied
    • More protein bound
    • When binding sites are saturated, then the free T4 effect will be seen
  • Levels of TSH will drop when adequate levels of T4 are reached

b. Thyroxine – T3 (Liothyronine sodium)

  • Na+ salt of T3
  • Used together with hydrocortisone (steroid) in hypothyroid coma

Course of action:

  • Faster action than T4
    • Less protein bound
    • More free drug, more effect
  • Clinical response about 4 times as rapid as T4, so more adverse effects
    • Therefore less used than T4

c. Combination of T3 & T4 (Liotrix)

  • Constituent of many commercial thyroid hormone in fixed ratio of T4:T3 –> 4:1
    • Ratio normally released in the body
    • More T4 as compared to T3 in the body
    • Mimics ratio of normal thyroid secretion
  • Expensive
  • No special advantage
    • The cheaper Levothyroxine T4 will be automatically converted to T3 in the body
    • Given T4 only should do the trick, no need for a combination


Clinical scenario:

A 25 y.o male presented with clinical features of bulging of both eyes, excessive sweating (hyperhydrosis), fine tremor of outstretched hands and loss of weight.

What is the likely diagnosis?

  • Hyperthyroidism
    • After ruling out cancer, anorexia, or salbutamol overdose (causes fine tremor)

What would the BP be like?

  • Increased.
    • Increased sympathetic activity due to increased metabolism

What is the condition of the bulging eyes called?

  • Exophthalmus
    • Proptosis: 1 eye bulging

The patient goes into thyroid storm. Name 2 drugs that you will use.

  • Iodide
  • Propanolol

What are their mechanisms of action?

  • Iodide inhibits organification & hormone release, also reduces conversion of T4 to T3
  • Propanolol is a beta blocker. It decreases the sympathetic activy & the blood pressure.

Iodide is used in cough syrup – TRUE?

Iodide increases T4-T3 – FALSE

Iodide causes rapid & acute thyrotoxicosis -FALSE

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