A comparison of thyroxine and desicatted thyroid in patients with primary hypothyroidism

Liothyronine and Desiccated Thyroid Extract in the Treatment of Hypothyroidism

Background: The basis for the treatment of hypothyroidism with levothyroxine (LT4) is that humans activate T4 to triiodothyronine (T3). Thus, while normalizing serum thyrotropin (TSH), LT4 doses should also restore the body's reservoir of T3. However, there is evidence that T3 is not fully restored in LT4-treated patients. Summary: For patients who remain symptomatic on LT4 therapy, clinical guidelines recommend, on a trial basis, therapy with LT4+LT3. Reducing the LT4 dose by 25 mcg/day and adding 2.5-7.5 mcg liothyronine (LT3) once or twice a day is an appropriate starting point. Transient episodes of hypertriiodothyroninemia with these doses of LT4 and LT3 are unlikely to go above the reference range and have not been associated with adverse drug reactions. Trials following almost a 1000 patients for almost 1 year indicate that similar to LT4, therapy with LT4+LT3 can restore euthyroidism while maintaining a normal serum TSH. An observational study of 400 patients with a mean follow-up of ∼9 years did not indicate increased mortality or morbidity risk due to cardiovascular disease, atrial fibrillation, or fractures after adjusting for age when compared with patients taking only LT4. Desiccated thyroid extract (DTE) is a form of combination therapy in which the LT4/LT3 ratio is ∼4:1; the mean daily dose of DTE needed to normalize serum TSH contains ∼11 mcg T3, but some patients may require higher doses. The DTE remains outside formal FDA oversight, and consistency of T4 and T3 contents is monitored by the manufacturers only. Conclusions: Newly diagnosed hypothyroid patients should be treated with LT4. A trial of combination therapy with LT4+LT3 can be considered for those patients who have unambiguously not benefited from LT4.

Management of Patients Symptomatically Unresponsive to Levothyroxine: Natural Desiccated Thyroid Extract or the Combination of Levothyroxine and Liothyronine? A Research Priority

Around 5-10% of hypothyroid patients continue to experience profound and sometimes disabling symptoms, including fatigue, depression and impaired cognition, in spite of being adequately replaced biochemically. The use of the combination of levothyroxine and liothyronine and natural desiccated thyroid extract is controversial for reasons of costs, a lack of evidence of additional benefit over levothyroxine alone, and potential safety concerns. Clinical guidelines caution against the use of both, and advise that only in exceptional cases may a short trial be considered. Natural desiccated thyroid extract is not licensed for use in the UK. However, key deficiencies in the existing evidence-base together with improved understanding of the pharmacology of levothyroxine resistance, indicates that now is the right time for a definitive clinical trial to address this important area of uncertainty.

Thyroxine replacement: a clinical endocrinologist's viewpoint

BACKGROUND

Hypothyroidism affects 2-5% of the general population. Patients with uncorrected disease suffer significant morbidity and have an increased risk of cardiovascular disease and neurocognitive impairment. Levothyroxine, the treatment of choice, is inexpensive, easy to administer and in most cases restores well-being while normalizing thyroid function. However, 30-50% of individuals on levothyroxine are either over-treated or under-treated and others remain dissatisfied with treatment despite achieving thyroid hormone concentrations within the laboratory reference interval.

METHODS

This review is based on a systematic search of the literature for controlled trials, systematic reviews, guideline papers and cohort studies addressing best practice in thyroid hormone replacement.

RESULTS

Recent decades have seen improvements in patient management strategies driven by a better appreciation of levothyroxine pharmacokinetics. However, aspects of therapy such as the optimal timing of medication, strategies to overcome treatment non-adherence and target thyroid stimulating hormone concentrations in pregnancy and in patients with differentiated thyroid cancer remain challenging. Furthermore, there is now a substantial body of literature on common genetic variations in the deiodinases and thyroid hormone transporters and their role in the local regulation of thyroid hormone delivery. The benefits of combination therapy with liothyronine and levothyroxine are uncertain, and while it is theoretically probable that subsets of genetically predisposed individuals will benefit from combination therapy the existing evidence is as yet limited.

CONCLUSION

Despite the availability of thyroid hormone replacement for more than a century, there are still substantial challenges in practice and opportunities to improve treatment outcomes.

Desiccated thyroid extract compared with levothyroxine in the treatment of hypothyroidism: a randomized, double-blind, crossover study

CONTEXT

Patients previously treated with desiccated thyroid extract (DTE), when being switched to levothyroxine (L-T₄), occasionally did not feel as well despite adequate dosing based on serum TSH levels.

OBJECTIVE

Our objective was to investigate the effectiveness of DTE compared with L-T₄ in hypothyroid patients.

DESIGN AND SETTING

We conducted a randomized, double-blind, crossover study at a tertiary care center.

PATIENTS

Patients (n = 70, age 18-65 years) diagnosed with primary hypothyroidism on a stable dose of L-T₄ for 6 months were included in the study.

INTERVENTION

Patients were randomized to either DTE or L-T₄ for 16 weeks and then crossed over for the same duration.

OUTCOME MEASURES

Biochemical and neurocognitive tests at baseline and at the end of each treatment period were evaluated.

RESULTS

There were no differences in symptoms and neurocognitive measurements between the 2 therapies. Patients lost 3 lb on DTE treatment (172.9 ± 36.4 lb vs 175.7 ± 37.7 lb, P < .001). At the end of the study, 34 patients (48.6%) preferred DTE, 13 (18.6%) preferred L-T₄, and 23 (32.9%) had no preference. In the subgroup analyses, those patients who preferred DTE lost 4 lb during the DTE treatment, and their subjective symptoms were significantly better while taking DTE as measured by the general health questionnaire-12 and thyroid symptom questionnaire (P < .001 for both). Five variables were predictors of preference for DTE.

CONCLUSION

DTE therapy did not result in a significant improvement in quality of life; however, DTE caused modest weight loss and nearly half (48.6%) of the study patients expressed preference for DTE over L-T₄. DTE therapy may be relevant for some hypothyroid patients.

Raising the decontamination level for thyroid hormone ingestions

Ninety-two pediatric cases (age < or = 6 years) of acute thyroid hormone ingestions that were reported over a 20-month period to one American Association of Poison Control Centers (AAPCC)-certified regional poison center were reviewed to determine whether significant toxicity in pediatric patients is associated with acute ingestions of < or = 5 mg levothyroxine (T4) equivalent of thyroid hormone and the highest-tolerated milligram-per-kilogram dose. Parameters evaluated included patient weight, amount ingested, drug preparation, treatment type, outcome, management site, and relevant comments. Eight patients had mild symptoms requiring no specific medical intervention and there was no correlation between the amount ingested and symptoms observed. The maximum T4 dose ingested without gastrointestinal decontamination was 0.13 mg/kg (2 mg). A literature review was also conducted. Only one case of moderate toxicity with a dose of less than 5 mg T4 equivalent was found in the literature review. Significant toxicity is not expected with ingestions of less than or equal to 5 mg T4 equivalent of thyroid hormone.

Aging and the thyroid. Decreased requirement for thyroid hormone in older hypothyroid patients

In 84 patients, aged 23 to 84, with primary hypothyroidism, the daily dose of thyroxine needed to lower the serum thyrotropin level into the normal range was significantly less in older patients than in younger ones (p less than 0.01). Most of the difference between middle-aged (40 to 60 years) and older patients (greater than 60 years) was due to a decrease in the required dose in men; there was no difference in the dose needed by women in these age groups. Previous hyperthyroidism did not affect the dose of thyroxine required; it is unlikely that residual autonomous thyroid tissue affected the dose. Although the wide range of doses needed precludes use of these data in calculating a dose of thyroxine for an individual patient, doses of 100 micrograms per day or less were common in patients over age 40, and a few patients over age 60 needed 50 micrograms per day or less. Thus, (1) there is a sound physiologic basis for the common practice of using low doses of thyroxine, e.g., 25 micrograms per day, as initial therapy in older hypothyroid patients and (2) it may be reasonable to reassess the dose of thyroxine after several years in older patients.

Bioavailability of thyroid hormones from oral replacement preparations

We evaluated gastrointestinal absorption in normal subjects of T4 and T3 from synthetic T3 tablets (Cytomel, SKF), desiccated thyroid tablets (Armour), thyroglobulin tablets (Proloid, Warner-Chilcott) and synthetic L-T4 tablets (Synthroid, Flint and Levothroid, Armour). Measurements of serum T4 and T3 concentrations and free hormone indices were made at multiple times after tablet ingestion, and T3 content in tablets was measured by radioimmunoassay. The time to peak serum T3, and the 26 hr intergrated increment in serum T3, Corrected for the amount if T3 ingested, were not significantly different for 75 micrograms of synthetic T3, 6 grains of desiccated thyroid (containing 99 micrograms T3) and 5 grains of thyroglobulin (containing 90 micrograms T3), the mean integrated increment values for the biological preparations being within 12% of those for synthetic T3. The peak serum T4 concentration, the time to peak T4, and 48 hr integrated increments in serum T4 and T3 were similar after 3 mg of Synthroid and Levothroid. The mean peak serum Free T3 Index after 75 micrograms T3, 500, was much higher than the mean peak Free T3 Index after 3 mg T4, 290. The time to peak Free T3 Index was much less after 75 micrograms T3, 2 hr, than the time to peak after 3 mg T4, 2 days. These results indicate that the time course and extent of T3 absorption do not differ, whether the T3 is given as the synthetic iodothyronine or as part of the thyroid protein, thyroglobulin. This approach appears to be useful in determining bioavailability of thyroid hormones from oral preparations and to assess the possibility of thyroid hormone malabsorption.