Levothyroxine

(levothyroxine)

Brands

Soloxine, Synthroid, Leventa

Availability

Tablets: Available in a variety of tablet sizes in both veterinary and human products
Solution: Oral solution 1mg/mL in 30 mL bottles under the brand Leventa Oral Solution

Pharmacology

Levothyroxine sodium is a synthetic thyroid hormone that is given, as thyroid replacement therapy, in conditions where the thyroid gland is unable to produce hormones in adequate amounts.

The thyroid gland makes and secretes the thyroid hormones: T4 (thyroxine) and T3 (triiodothyronine). These hormones affect many of the body’s physiologic processes by speeding up or slowing down the body’s metabolic rate of the brain, heart, liver, and other organs. It does this by: affecting how fats, proteins and carbohydrates are metabolized, increasing and decreasing consumption of oxygen, affecting body temperature, the heart’s rate and volume, as well as development, growth and maturity of the body, and more. Exactly how thyroid hormones exert these many effects are not well known, but the action occurs at the cellular level.

The thyroid gland works together with the pituitary gland. The pituitary gland make’s, stores and releases TSH (thyroid-stimulating hormone) which causes the thyroid gland to secrete T3 and T4. A high TSH level in the blood means little of the thyroid hormone is being released, and a low TSH level level in blood means to much is being released.

Levothyroxine, the synthetic hormone, replaces the endogenous thyroxine (T4) when it’s not able to be produced by the thyroid in adequate amounts.

Thyroid hormones do not readily cross the placenta; though it has been shown that some transfer does occur in human fetuses. The thyroid hormones are also shown to be minimally distributed into breast milk.
The use of exogenous levothyroxine has not, to this point, been evaluated in pregnant and lactating animals.

The thyroid hormones, T3 and T4, are metabolized principally in the liver through sequential deiodination. The thyroid hormones are also metabolized via conjugation with glucuronides (a binding to make substances more water-soluable for easier excretion through and from the body) and sulfates and excreted directly into the bile and gut where they undergo enterohepatic recirculation.

In clinical studies the active sites of absorption of orally administered thyroid hormone, in animals, appears to be the proximal and mid-jejunum with little being absorbed in the duodenum and none from the distal colon or stomach.

Levothyroxine is primarily eliminated by the kidneys, with a portion being eliminated unchanged in feces.

Indications

Used as a lifelong medication in the treatment of hypothyroidism (underactive thyroid).

Drug Interactions or Contraindications

Possible interactions may occur with antidepressants, digoxin, epinephrine, insulin, ketamine, and norepinephrine.
For pets receiving sucralfate (Carafate) or aluminum antacids (e.g., Maalox, Mylanta), give 4 hours before or after giving levothyroxine.
Levothyroxine is contraindicated in animals with thyrotoxicosis, or untreated adrenal insufficiency.
Use with caution in animals being treated for cardiac issues, diabetes, hypoadrenocorticism on treatment, or in the very aged rat.

Adverse Reactions

CNS: drooling, restlessness, seizures

CV: tachycardia

GI: diarrhea, inappetence, polydipsia (PD)

GU: polyuria (PU)

Skin: fur thinning

Dosage Recommendations

*Note:
5 micrograms (mcg)/kg (equal to: 0.005 mg/kg), PO, q12hr ³⁴^, ⁴¹^, ⁴²^, ⁴⁴

or

10 micrograms (mcg)/kg (equal to: 0.01 mg/kg), PO, daily in divided doses ⁴⁰

Considerations

Due to bioavailability differences between brands it is advised to stay with a brand that is initiated rather than switching between brands during treatment.
Store at room temperature in light-resistant container. Discard any unused reconstituted solutions or those past their expiration date.

References

Soloxine (levothyroxine sodium) tablets. (2011, May 12). Retrieved July 25, 2016, from https://ca.virbac.com/pdfs/inserts/750029-01_soloxine_tablets_insert-web_12may11.pdf
Synthroid product monograph. (2014, December 22). Retrieved July 25, 2016, from http://www.abbott.ca/docs/SYNTHROID-PM-E.pdf