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Whats the difference between the various synthetic thyroid hormones?

Posted May 01 2011 12:00am

- synthetically prepared levo-isomer of thyroxine (T4)
- used for treatment of primary, secondary (pituitary), or tertiary (hypothalamic) hypothyroidism
- Can be used in treatment/reduction of goiter
- Preferred over thyroid and thyroglobulin because the hormonal content of levothyroxine is standardized and the drug effects are more predictable
- It exhibits all of the actions of endogenous thyroid hormone
- It only provides T4, which about 80% is de-iodinated to T3 and rT3 in the peripheral tssues. Since T3 is 3x as potent as T4, virtually all of the activity of T4 can be attributed to T3
- Used clinically since the 1950s
- Administered orally
- Slower onset of action and slow metabolic clearance
- Highly bound to thyroglobulin binding protein and albumin in the blood
- T ½ = 6-7 days
- Majorly absorbed in the jejunum and upper ileum (assuming no malabsorption, CHF, or diarrhea)
- Absorption is increased by fasting

- Principally T3 – synthetic sodium salt of endogenous thyroid hormone triiodothyronine T3
- Mimicks the biologically active form of the endogenous thyroid hormone T3 (T3 is 4x more active than T4, but lower serum levels are maintained)
- Treatment of hypothyroidism of any etiology, except
- transient hypothyroidism during the recovery phase of subacute thyroiditis,
- a TSH suppressant in the treatment of a euthyroid goiter
- Diagnostic agent in T3 suppression tests
- Oral dose or injection
- Within 4 hours, 95% of an oral dose is absorbed from the GI tract
- Quick onset of action
- Not as tightly bound to serum proteins
- Allows for rapid cellular penetration
- T ½ = 2.5 days
- Allows for faster dosage adjustments
- Can be used for patients who are allergic to dessicated thyroid
- More cardiotoxic than levothyroxine
- Recommended for treatment of myxedema coma
- Considered to be the most appropriate thyroid replacement hormone for long-term treatment of hypothyroidism
- However, a small randomized clinical trial suggested that there may be some mood and neuropsychological functions associated

- Mixture of 2 synthetic thyroid hormones T4 and T3 in a 4:1 ratio by weight
- the logic behind the 4:1 ratio is unclear, because in a euthyroid patient, the ratio of T4:T3 is much higher ranging from 17:1 outside the thyroid, to 70:1 for serum concentrations
- Oral synthetic thyroid hormone preparation used for replacement therapy in individuals with deficient intrinsic thyroid hormones
- Liotrix does not have any discernable advantage over levothyroxine alone in treatment of hypothyroidism
- But the advantage that it does have is the hormone content might be preferable to desiccated porcine thyroid hormone.
- Administered orally with variable absorption (can be increased during fasting)

Desiccated Thyroid
- Naturally occurring thyroid hormone derived from porcine (por•sine) thyroid glands
- Contains both levothyroxine and liothyronine = possesses all of the actions of endogenous thyroid hormones
- Used to treat hypothyroidism for - Primary atrophy of the gland
- Functional thyroid deficiency
- Decreased thyroid function 2° to surgery, radiation, antithyroid agents
- Pituitary hypothyroidism
- Hypothalamic hypothyroidism
- Cretinism (stunted physical and mental growth d/t lack of thyroid hormones)
- Largely has been replaced by synthetic levothyroxine (more reliable hormone content)
- Administered orally with a 50-75% bioavailability
- Believed to distribute predominantly to the liver and the kidney tissues
- T ½ is about 4 days

References 1.) Medscape Drug Monograph: Levothyroxine. May 23, 2011.
2.) Medscape Drug Monograph: Liotrix. May 23, 2011.
3.) Medscape Drug Monograph: Deiccated Thyroid. May 23, 2011.
4.) Medscape Drug Monograph: Liothyronine. May 23, 2011.

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