Levothyroxine dosing.

[Guest guest]

OK, so the patient comes in. "I can't take it first thing in the morning on an empty stomach". Doesn't matter the reason. So I say, "as long as you take it consitantly, the same way every day, that's OK". I feel it is certainly better than not taking it.

Am I wrong?

[Guest guest]

1/2 life is 3 to 4 days in hyperthyroidism, 9 to 10 days in hypothyroidism

One would think it might not matter what time of day that much.

But I suppose it does.

http://en.wikipedia.org/wiki/Synthroid

ca. 7 days (in hyperthyroidism 3-4 days, in hypothyroidism 9-10 days)

http://www.rxlist.com/synthroid-drug.htm

Table 1: Pharmacokinetic Parameters of Thyroid Hormones in Euthyroid Patients

Hormone

Ratio in Thyroglobulin

Biologic Potency

t¨ö (days)

Protein Binding (%)2

Levothyroxine (T4)

10 - 20

1

6-71

99.96

Liothyronine (T3)

1

4

¡Â 2

99.5

13 to 4 days in hyperthyroidism, 9 to 10 days in hypothyroidism2Includes TBG, TBPA, and TBA

Pharmacokinetics

Absorption- Absorption of orally administered T4 from the gastrointestinal (GI) tract ranges from 40% to 80%. The majority of the levothyroxine dose is absorbed from the jejunum and upper ileum. The relative bio availability of SYNTHROID tablets, compared to an equal nominal dose of oral levothyroxine sodium solution, is approximately 93%. T4 absorption is increased by fasting, and decreased in malabsorption syndromes and by certain foods such as soybean infant formula. Dietary fiber decreases bio availability of T4. Absorption may also decrease with age. In addition, many drugs and foods affect T4 absorption (see PRECAUTIONS: DRUG INTERACTIONS and Drug-Food Interactions).

Distribution- Circulating thyroid hormones are greater than 99% bound to plasma proteins, including thyroxine-binding globulin (TBG), thyroxine-binding pre albumin (TBPA), and albumin (TBA), whose capacities and affinities vary for each hormone. The higher affinity of both TBG and TBPA for T4 partially explains the higher serum levels, slower metabolic clearance, and longer half-life of T4 compared to T3. Protein-bound thyroid hormones exist in reverse equilibrium with small amounts of free hormone. Only unbound hormone is metabolically active. Many drugs and physiologic conditions affect the binding of thyroid hormones to serum proteins (see PRECAUTIONS: DRUG INTERACTIONS and Drug-Laboratory Test Interactions). Thyroid hormones do not readily cross the placental barrier (see PRECAUTIONS, Pregnancy).

Metabolism- T4 is slowly eliminated (see Table 1). The major pathway of thyroid hormone metabolism is through sequential deiodination. Approximately eighty-percent of circulating T3 is derived from peripheral T4 by monodeiodination. The liver is the major site of degradation for both T4 and T3, with T4 deiodination also occurring at a number of additional sites, including the kidney and other tissues. Approximately 80% of the daily dose of T4 is deiodinated to yield equal amounts of T3 and reverse T3 (rT3). T3 and rT3 are further deiodinated to diiodothyronine. Thyroid hormones are also metabolized via conjugation with glucuronides and sulfates and excreted directly into the bile and gut where they undergo enterohepatic recirculation.

Elimination- Thyroid hormones are primarily eliminated by the kidneys. A portion of the conjugated hormone reaches the colon unchanged and is eliminated in the feces. Approximately 20% of T4 is eliminated in the stool. Urinary excretion of T4 decreases with age. Locke, MD

From: [mailto: ] On Behalf Of nancy blakeSent: Thursday, November 20, 2008 8:38 AMTo: IMP GroupSubject: Levothyroxine dosing.

OK, so the patient comes in. "I can't take it first thing in the morning on an empty stomach". Doesn't matter the reason. So I say, "as long as you take it consitantly, the same way every day, that's OK". I feel it is certainly better than not taking it.

Am I wrong?

[Guest guest]

Common sense, job security for us to keep us from being replaced by a computer programmed with all the latest 'best practices' data ;-)

-- Lynette I Iles MD 301 South Iowa Ste 2Washington IA 52353 Flexible Family Care'Modern medicine the old-fashioned way' This e-mail and attachments may contain information which is confidential and is only for the named addressee. If you have received this email in error, please notify the sender immediately and delete it from your computer.