Uses of Thyroid Hormone

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Chapter: Essential pharmacology : Thyroid Hormones And Thyroid Inhibitors

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:

 

Cretinism

 

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.

 

Adult Hypothyroidism

 

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.

 

Myxoedema Coma

 

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.

 

Nontoxic Goiter

 

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.

 

Thyroid Nodule

 

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.

 

Papillary Carcinoma Of Thyroid

 

It is often responsive to TSH. In nonresectable cases, full doses of T4 suppress TSH production and may induce temporary regression.

 

Empirical Uses

 

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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