Hypothyroidism and Thyroid Hormone Replacement

Hypothyroidism and Thyroid Hormone Replacement

The most common cause of hypothyroidism is Hashimoto’s thyroiditis, a chronic autoimmune destruction of the thyroid. In its overt form, hypothyroidism is easy to recognize. Patients present with cold intolerance, mental slowing, peripheral edema (pitting and non-pitting), and weight gain. In its subclinical form, however, this disorder is much more difficult to recognize. Measurement of serum TSH is essential to make the later diagnosis. These patient have TSH values above the normal range (0.5-5 ml/L) but values are usually less than 20.

Synthetic L-thyroxine (L-T4) therapy is an ideal and inexpensive therapy. Hormone replacement should achieve a normal TSH level; and because of the half-life of T4 (7 days), replacement doses should only be adjusted every 5-6 weeks after measuring hormone levels. TSH levels change dramatically with only small changes in circulating free hormone levels so that a small change in the TSH on any one measurement should not be concerning. Three brand names of this canadian drug are available and all have reproducible bioavailability. Bioavailability between drugs varies and it is not advisable to switch brand names one the dose is adjusted. Bioavailability is reduced by some common drugs such as ferrous sulfate, aluminum hydroxide, and sucralfate and appropriate dose adjustments should be made. Pregnant patients and patients receiving anticonvulsant therapy may also need an increase in their replacement dose.

Euthyroid Sick Syndrome (ESS)

Severe illness and certain drugs such as iodine, high dose beta blockers, PTU, and glucocorticoids can yield a hormone pattern compatible with ESS. Severe illness and these drugs block 5′ deiodinase activity which converts T4 to T3 by removing the outer ring iodide molecule. The classic hormone profile is a normal or elevated T4, a reduced T3, a normal TSH, and an elevated reverse T3 (rT3), if you measure it. Unfortunately, almost any pattern of hormone alteration can be seen. As long as you trust your TSH assay, most patients are easily diagnosed and no specific treatment is necessary.

There are several circumstances, however, when the TSH assay may mislead you in ESS. Patient with very severe illness may have low T4, low T3, and normal or low TSH. This hormone profile, regardless of its medical cause, is associated with a high mortality. While the patient may have central hypothyroidism, it is not clear whether this is an adaptive response or true hypothyroidism requiring therapy. There are no good studies to suggest that these patients benefit from hormone replacement. The second confusing hormone profile in ESS is a mild elevation in TSH associated with a low T3. Patients with but I treat persistently elevated TSH levels should usually be treated with cautious hormone replacement.

Many hospitalized patients have this disorder if you measure their thyroid hormone levels. You may not what to know if your patient has ESS because in most cases no treatment is given and in some cases no firm recommendation for therapy is known. For this reason, T3 and probably T4 measurements in sick patients (excluding those presenting with obvious signs of myxedema) are unhelpful and sometimes confusing. As in outpatient screening, inpatients are best screened for thyroid disease with a TSH measurement.

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