LEVOTHYROXINE Oral solution Ref.[6943] Active ingredients: Levothyroxine

Source: Medicines & Healthcare Products Regulatory Agency (GB)  Revision Year: 2019  Publisher: Colonis Pharma Limited, Quantum House, Hobson Industrial Estate, Burnopfield, County Durham, NE16 6EA

Pharmacodynamic properties

Thyroid hormones
ATC Code: H03AA01

Thyroxine (T4) is a naturally occurring hormone containing iodine, produced by the thyroid gland. It is converted to its more active principle triiodothyronine (T3) in the peripheral tissues. Receptors for T3 are found on cell membranes, mitochondria and cell nuclei. Thyroid hormones are required for normal growth and development of the body, especially the nervous system. They increase the basal metabolic rate of the whole body and have stimulatory effects on the heart, skeletal muscle, liver and kidney.

The synthetic levothyroxine contained in Levothyroxine Oral Solution is identical in effect with the naturally occurring thyroxine secreted by the thyroid.

Pharmacokinetic properties

Levothyroxine sodium is incompletely and variably absorbed from the gastrointestinal tract. Levothyroxine is extensively metabolised in the thyroid, liver, kidney and anterior pituitary. Some enterohepatic re-circulation occurs. Part of the levothyroxine is metabolised to triiodothyronine. Levothyroxine is excreted in the urine and faeces, partly as free drug and partly as conjugates and de-iodinated metabolites.

It has a half life of 7 days but this may be shortened or prolonged depending on the disease condition. Levothyroxine is almost completely bound to plasma protein, mainly thyroxine binding globulin, with approx. 0.03% of levothyroxine unbound. The unbound levothyroxine is converted to triiodothyronine.

There are four main pathways of metabolism:

  1. Deiodination to triiodothyronine (active) - T3 or to reverse triiodothyronine (inactive). Further deiodination of T3 leads to the formation of thyroacetic acid.
  2. Deamination to the tetrone.
  3. Conjugation to the glucoronide or sulphate.
  4. Ether bond cleavage to diiodotyrosines.

The most important metabolic pathway is deiodination. Between 30-55% of the levothyroxine dose is excreted in the urine and 20-40% in the faeces.

Preclinical safety data

Not applicable since levothyroxine has been used in clinical practice for many years and its effects in man are well known.

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