The most important uses of thyroid hormone are as replacement therapy in deficiency states:
USES
The
most important uses of thyroid hormone are as replacement therapy in deficiency states:
It is due to failure of thyroid development or a defect in hormone synthesis
(sporadic cretinism) or due to extreme iodine deficiency (endemic cretinism).
It is usually detected during infancy or childhood. Treatment with thyroxine
(8–12 μg/kg) daily should be
started as early as possible, because mental retardation that has already
ensued is only partially reversible. Response is dramatic: physical growth and
development are restored and further mental retardation is prevented.
This is one of the commonest endocrine disorders which develops
as a consequence of thyroiditis, thyroidectomy; may accompany simple goiter if
iodine deficiency is severe, or may be idiopathic. Important drugs that can
cause hypothyroidism are 131I, iodides, lithium and amiodarone. Treatment with
T4 is most gratifying. It is often wise to start with a low dose—50 μg of l-thyroxine daily
and increase every 2–3 weeks to an optimum of 100–200 μg/day (adjusted by
clinical response and serum TSH levels). Further dose adjustments are made at
4–6 week intervals needed for reaching steadystate. Individualization of proper
dose is critical, aiming at normalization of serum TSH levels. Increase in dose
is mostly needed during pregnancy.
Subclinical Hypothyroidism characterized by euthyroid status and
raised TSH level (>10 mU/L should be treated with T4 if other
cardiovascular risk factors are present; otherwise replacement therapy is
optional.
It is an emergency;
characterized by progressive mental deterioration due to acute hypothyroidism:
carries significant mortality. Rapid thyroid replacement is crucial. Though
liothyronine (T3) acts faster, its use is attended by higher risk of
cardiac arrhythmias, angina, etc. Drug of choice is l-thyroxine (T4)
200– 500 μg i.v. followed by 100
μg i.v. OD till oral
therapy can be instituted. Some authorities recommend adding low dose i.v. T3
10 μg 8 hourly in younger
patients with no arrhythmia or ischaemia. Alternatively oral T4 500 μg loading dose
followed by 100–300 μg daily may be used, but in severe hypothyroidism, oral
absorption is delayed and inconsistent.
Corticosteroids to
cover attendant adrenal insufficiency, ventilatory and cardiovascular support,
correction of hyponatraemia, hypoglycaemia, etc. are the other measures.
It may be endemic or sporadic. Endemic is due to iodine
deficiency which may be accentuated by factors present in water (excess
calcium), food or milk (goitrin, thiocyanates). A defect in hormone synthesis
may be responsible for sporadic cases. In both types, deficient production of
thyroid hormone leads to excess TSH → thyroid enlarges, more efficient trapping of
iodide occurs and probably greater proportion of T3 is synthesized → enough hormone to
meet peripheral demands is produced. Thus, treatment with T4 is in
fact replacement therapy in this condition also, despite no overt
hypothyroidism. Full maintenance doses must be given. Most cases of recent
diffuse enlargement of thyroid regress. Longstanding goiters with degenerative
and fibrotic changes and nodular goiter respond poorly or not at all. Therapy
may be withdrawn after a year or so in some cases if adequate iodine intake is
ensured. Others need lifelong therapy.
Endemic goiter and
cretinism due to iodine deficiency in pregnant mother is preventable by
ensuring daily ingestion of 150–200 μg of iodine. This is best achieved by iodizing
edible salt. In India iodization of table salt (100 μg iodine/g salt) is
required under the National Programme, but recently mandatory iodization rule
has been withdrawn.
Certain benign functioning nodules regress when TSH is suppressed by T4
therapy. Nonfunctional nodules and those nonresponsive to TSH (that are
associated with low TSH levels) do not respond. T4 therapy should be
stopped if the nodule does not decrease in size within 6 months and when it
stops regressing.
It is often responsive to TSH. In nonresectable cases,
full doses of T4 suppress TSH production and may induce temporary
regression.
T4 has been sometimes
used in the following conditions
without any rationale; response is unpredictable.
Refractory
anaemias.
Menstrual disorders,
infertility not corrected by usual treatment.
/Chronic/non healing
ulcers. Obstinate constipation.
Thyroxine
is not recommended for obesity and as a hypo-cholesterolemic agent.
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