Levothyroxine & RAI - how soon to restart meds (Dr. Ain reposts)

[Guest guest]

> .... Endo explained why I can't start my synthroid till 10 days after

RAI...

> he said the RAI needs all that time to do its job.....

Dear ThyCa Members:

It is my practice to resume full levothyroxine treatment doses

immediately after discharging patient from the radiation isolation rooms

(usually be 24 hours after I-131 administration). First, the TSH will

remain

quite elevated for at least 2 weeks after starting levothyroxine.

Second, all of the therapeutic I-131 uptake into tumor takes place within

the first 3 days of taking the radioactive iodine. Lastly, there is no

conceivable

reason to delay a patient's return to a levothyroxine-repleted state.

[... and ....]

> This is taken from an article: " The trick is to clamp, or shut off the

>TSH when radioiodide uptake and retention is maximum in the lesions being

>treated. " They use a full replacement dose of Cytomel plus a double dose

of

>thyroxine for four days.

Although I certainly advocate starting patients back on levothyroxine

therapy just upon discharge from radiation isolation by 24 hours after

receiving I-131 therapy, the particular reason espoused above and discussed

at that website is not the reason. The article accessed via that web site

is NOT PEER-REVIEWED, it is not a publication in a medical journal, and it

is merely one physician's advertised approach to thyroid cancer.

In fact, in my professional opinion, there is no validity to the idea of

" clamping the TSH " particularly since even massive thyroid hormone doses

are not capable of suppressing TSH for at least several weeks. This is an

interesting " idea " , but definitely not SCIENCE and not something validated

by any known research.

I start patients back on their full levothyroxine dose and utilize a

co-administered Cytomel dose which is planned to TAPER OFF over

2-3 weeks as the T4 level rises in the patient's blood. Cytomel is not

advocated past that initiation period.

In obtaining information over the WEB, it is critical to understand that

ANYONE can say ANYTHING on a web site, and the mere " publication " of

something on a web site does not validate its contents. This particular

case appears to be one in which a correct plan is presented for wholly

INCORRECT reasons.

[... and ...]

> [nuc med tech] said they don't have people start back on meds

> until after their scan, which is 10 days after receiving RAI.

> He said if I started back on meds it would interfere with the uptake.

The information requested above is far from new, in fact, it is

ancient. Even in patients with intravenous infusions of

levothyroxine supplemented with an initial dose of 500-800 mcg, it

takes at least 4 days to lower the TSH by 50% and at least one week

to lower the TSH below 30. [Ridgway EC, McCammon JA, Benotti J &

Maloof F (1972) Acute metabolic responses in myxedema to large doses

of intravenous L-thyroxine. Ann Intern Med 77, 549-555.]

Administration of daily intravenous levothyroxine doses of 100 mcg

required more than one week to reduce the TSH by 50%. [Ladenson PW,

Goldenheim PD, DS, MA & Ridgway EC (1982) Early

peripheral responses to intravenous L-thyroxine in primary

hypothyroidism. Am J Med 73, 467-474.] Elevated TSH is the mechanism

of stimulation of thyroid cancer cells to take up I-131 and the

reason for with-holding levothyroxine.

There is no longer sufficient I-131 circulating in the

bloodstream after 48 hours from swallowing I-131 to contribute to the

treatment of the tumor. Also, as shown above, starting levothyroxine

does not suppress the TSH significantly for several days. For this

reason, I start patients back on their full levothyroxine daily dose

at 24 hours after administering their radioactive iodine.

This is another example of the " practice of medicine " differing

significantly from the " science of medicine. " I strive to make my

own practice to be consistent with science and biology, as all

physicians should.

[... and ...]

>I remember a discussion between 2 Oncologists here about the possibility

> of administering the T4 24h after the RAI, something to do with dropping

> the TSH thereby locking the RAI in the thyroid tissue.

>

>A) I'm not sure that I remember this right

> I'm not sure whether this is 'off-the-wall theory' or 'known fact'

>c) I don't know whether this has ever been tested on not.

>

>Would one of the Docs like to comment?

Dear ThyCa Members:

I start levothyroxine at 24 hours after administration of the I-131

therapy dose for the following reasons:

1) Despite starting full dose levothyroxine (plus a tapering dose

of Cytomel) at that time, the TSH will remain severely elevated for

at least 2 more weeks;

2) There is no valid reason to delay a patient's recovery from

hypothyroidism as soon as possible.

The suggested " lock-in " concept is pure hokum (bovine feces).

**************PLEASE BE ADVISED*********************

THE INFORMATION CONTAINED IN THIS COMMUNICATION IS INTENDED

FOR EDUCATIONAL PURPOSES ONLY. IT IS NOT INTENDED, NOR SHOULD

IT BE CONSTRUED, AS SPECIFIC MEDICAL ADVICE OR DIRECTIONS. ANY

PERSON VIEWING THIS INFORMATION IS ADVISED TO CONSULT THEIR OWN

PHYSICIAN(S) ABOUT ANY MATTER REGARDING THEIR MEDICAL CARE.

************************************************

B. Ain, M.D.

Associate Professor of Internal Medicine

Director, Thyroid Nodule & Oncology Clinical Service

Director, Thyroid Cancer Research Laboratory

Division of Endocrinology and Molecular Medicine

Department of Internal Medicine, Room MN524

University of Kentucky Medical Center

800 Rose Street, Lexington, Kentucky 40536-0298

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