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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.
For pets receiving sucralfate (Carafate) or aluminum antacids (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, hypoadrenocortisim on treatment, or in the very aged rat.
GI: diarrhea, inappetence, polydipsia (PD)
GU: polyuria (PU)
Skin: fur thinning
5 micrograms/kg (equal to: 0.005mg/kg), PO, q12hrs 34
10 micrograms/kg (equal to: 0.01mg/kg), PO, daily in divided doses 40
Posted on July 23, 2016, 11:19,
Last updated on October 11, 2016, 06:52
| Endocrine Hormones