on 4/6/02 11:55 AM, Strayze at [log in to unmask] wrote:

> To my knowledge beta blockers do not suppress the thyroid gland per se and
> its production of thyroid hormone.  Beta blockers are prescribed for high
> blood pressure, to slow down tachyarrhythmia, and to suppress certain but
> not all SYMPTOMS of thyrotoxicity (only the adrenergic symptoms of high
> blood pressure, tachyarrhthmia), whether it be endogenous or iatrogenic.
 ~~~~~~~~~~~

Yes, you are right.  Beta blocking drugs are not considered antithyroid
drugs per-se (in that they do not inhibit synthesis or secretion of thyroid
hormones by the thyroid gland).  Continued use for patients taking T3
however depletes excess T3 from the body very rapidly.

Beta blockers nullify the B-adrenergic effects of thyroid hormone, and it is
largely these effects that mediate the metabolism acceleration from thyroid
hormone.

Beta blockers are important for treating some symptoms of hyperthyroidism.
Propranolol, a B1 and B2 antagonist, tends to inhibit conversion by
deiodinase of T4 to T3 and in this sense it may be considered an antithyroid
drug.  This effect may be desirable in treatment of hyperthyroidism, but
when attempting to block acute tissue overstimulation from excessive doses
of exogenous thyroid hormone, the clinician may prefer not to block
deiodination.

Refetoff, Weiss, and Usala recommend atenolol, a B1 antagonist that inhibits
T4 to T3 conversion less than propranolol and has the benefit of a longer
duration of action.  Very little atenolol penetrates the blood/brain barrier
and as a result its use over time may have fewer adverse central nervous
system effects than propranolol, which readily passes the blood/brain
barrier.

The drug may have a profound hypotensive effect and should be used with
caution in fibromyalgia/chronic fatigue syndrome patients, many of whom are
already hypotensive.  Fibromyalgia patients should NOT use any beta blocking
drug except for ACUTE management of thyrotoxicosis due to *excess* intake of
thyroid hormone.  Using them over a prolonged time may induce or intensify
fibromyalgia symptoms in many patients.  And we know hypothyroid patients
often have fibromyalgia.

As you yourself have said:

"If you are on beta blockers for any reason (there are many different
brands), for say, performance anxiety, hypertension, to slow down one's
atrial fibrillation, etc. etc., many of the thyrotoxicity symptoms will be
masked.  In this case a thyroid patient on replacement therapy + beta
blockers should have more frequent checks (than 1/yr or 1/2yr) of thyroid
function, so that one will not permanently lose bone density."

Joan McPhee, not an M.D.
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