Evaluation of the therapeutic efficacy of different levothyroxine preparations in the treatment of human thyroid disease

Development of a Rat Model for Evaluating Thyroid-Stimulating Hormone Suppression after Total Thyroidectomy

BACKGROUND

We developed an animal model for evaluating thyroid-stimulating hormone (TSH) suppression therapy after total thyroidectomy in rats.

METHODS

Sixty Wistar rats were randomly divided into 6 groups, including a sham-operated group (SO group), a total thyroidectomy group (TD group), and a L-thyroxine (L-T4) treatment I group (TS-I group), II group (TS-II group), III group (TS-III group), and IV group (TS-IV group) (in which rats were treated with 1.4, 1.6, 1.8, and 2.0 μg/100 g body weight, respectively) after total thyroidectomy.

RESULTS

HE staining in the TD group and all L-T4-treated rats showed that the resected tissue was normal thyroid gland. No residual thyroid tissue was found in neck tissue of the cross-section of the thyroid gland. Serum T3 levels in the TS-II group were not significantly different from those in the SO group, whereas serum T4 levels were slightly higher than those in the SO group, and serum TSH levels were slightly lower.

CONCLUSIONS

Rats subcutaneously injected with L-T4 1.6 μg/100 g body weight for 15 days after total thyroidectomy were suitable as an animal model for TSH suppression therapy.

Levothyroxine sodium pentahydrate tablets - formulation considerations

Even though levothyroxine sodium pentahydrate tablets have been in the market since 1955, there continue to be recalls due to sub potency. We have comprehensively reviewed the factors affecting its stability in solid oral dosage forms. A compilation of marketed formulation compositions enabled the identification of the potential 'problem excipients'. Two excipient properties, hygroscopicity and microenvironmental acidity, appeared to be responsible for inducing drug instability. In drug products, depending on the formulation composition and storage conditions, the pentahydrate can dehydrate to highly reactive levothyroxine sodium monohydrate, or undergo salt disproportionation to the free acid form of the drug. The USP assay method (HPLC based) is insensitive to these different physical forms of the drug. The influence of physical form of levothyroxine on its chemical stability is incompletely understood. The USP has five product-specific dissolution tests reflecting the complexity in its evaluation.

Oral levothyroxine therapy postbariatric surgery: Biopharmaceutical aspects and clinical effects

BACKGROUND

Bariatric surgery can lead to changes in the oral absorption of many drugs. Levothyroxine is a narrow therapeutic drug for hypothyroidism, a common condition among patients with obesity.

OBJECTIVE

The purpose of this work was to provide a mechanistic overview of levothyroxine absorption, and to thoroughly analyze the expected effects of bariatric surgery on oral levothyroxine therapy.

METHODS

We performed a systematic review of the relevant literature reporting the effects of bariatric surgery on oral levothyroxine absorption and postoperative thyroid function. A PubMed search for relevant keywords resulted in a total of 14 articles reporting levothyroxine status before versus after bariatric surgery.

RESULTS

Different mechanisms may support opposing trends as to levothyroxine dose adjustment postsurgery. On the one hand, based on impaired drug solubility/dissolution attributable to higher gastric pH as well as reduced gastric volume, compromised levothyroxine absorption is expected. On the other hand, the great weight loss, and altered set-point of thyroid hormone homeostasis with decreased thyroid-stimulating hormone after the surgery, may result in a decreased dose requirement.

CONCLUSIONS

For patients after bariatric surgery, close monitoring of both the clinical presentation and plasma thyroid-stimulating hormone and T4 levels is strongly advised. Better understanding and awareness of the science presented in this article may help to avoid preventable complications and provide optimal patient care.

Amiodarone-induced thyroid dysfunction

Amiodarone is an effective medication for the treatment of cardiac arrhythmias. Originally developed for the treatment of angina, it is now the most frequently prescribed antiarrhythmia drug despite the fact that its use is limited because of potential serious side effects including adverse effects on the thyroid gland and thyroid hormones. Although the mechanisms of action of amiodarone on the thyroid gland and thyroid hormone metabolism are poorly understood, the structural similarity of amiodarone to thyroid hormones, including the presence of iodine moieties on the inner benzene ring, may play a role in causing thyroid dysfunction. Amiodarone-induced thyroid dysfunction includes amiodarone-induced thyrotoxicosis (AIT) and amiodarone-induced hypothyroidism (AIH). The AIT develops more commonly in iodine-deficient areas and AIH in iodine-sufficient areas. The AIT type 1 usually occurs in patients with known or previously undiagnosed thyroid dysfunction or goiter. The AIT type 2 usually occurs in normal thyroid glands and results in destruction of thyroid tissue caused by thyroiditis. This is the result of an intrinsic drug effect from the amiodarone itself. Mixed types are not uncommon. Patients with cardiac disease receiving amiodarone treatment should be monitored for signs of thyroid dysfunction, which often manifest as a reappearance of the underlying cardiac disease state. When monitoring patients, initial tests should include the full battery of thyroid function tests, thyroid-stimulating hormone, thyroxine, triiodothyronine, and antithyroid antibodies. Mixed types of AIT can be challenging both to diagnose and treat and therapy differs depending on the type of AIT. Treatment can include thionamides and/or glucocorticoids. The AIH responds favorably to thyroid hormone replacement therapy. Amiodarone is lipophilic and has a long half-life in the body. Therefore, stopping the amiodarone therapy usually has little short-term benefit.

Generic and brand-name L-thyroxine are not bioequivalent for children with severe congenital hypothyroidism

CONTEXT

In the United States, generic substitution of levothyroxine (L-T(4)) by pharmacists is permitted if the formulations are deemed to be bioequivalent by the Federal Drug Administration, but there is widespread concern that the pharmacokinetic standard used is too insensitive.

OBJECTIVE

We aimed to evaluate the bioequivalence of a brand-name L-T(4) (Synthroid) and an AB-rated generic formulation (Sandoz, Princeton, NJ) in children with severe hypothyroidism.

DESIGN

This was a prospective randomized crossover study in which patients received 8 weeks of one L-T(4) formulation followed by 8 weeks of the other.

SETTING

The setting was an academic medical center.

PATIENTS

Of 31 children with an initial serum TSH concentration >100 mU/L, 20 had congenital hypothyroidism (CH), and 11 had autoimmune thyroiditis.

MAIN OUTCOME MEASURES

The primary endpoint was the serum TSH concentration. Secondary endpoints were the free T(4) and total T(3) concentrations.

RESULTS

The serum TSH concentration was significantly lower after 8 weeks of Synthroid than after generic drug (P = .002), but thyroid hormone levels did not differ significantly. Subgroup analysis revealed that the difference in TSH was restricted to patients with CH (P = .0005). Patients with CH required a higher L-T(4) dose (P < .0004) and were younger (P = .003) but were not resistant to thyroid hormone; 15 of 16 CH patients had severe thyroid dysgenesis or agenesis on imaging. The response to generic vs brand-name preparation remained significant when adjusted for age.

CONCLUSIONS

Synthroid and an AB-rated generic L-T(4) are not bioequivalent for patients with severe hypothyroidism due to CH, probably because of diminished thyroid reserve. It would therefore seem prudent not to substitute L-T(4) formulations in patients with severe CH, particularly in those <3 yr of age. Our results may have important implications for other severely hypothyroid patients in whom precise titration of L-T(4) is necessary.

TSH-based protocol, tablet instability, and absorption effects on L-T4 bioequivalence

BACKGROUND

FDA Guidance for pharmacokinetic (PK) testing of levothyroxine (L-T(4)) for interbrand bioequivalence has evolved recently. Concerns remain about efficacy and safety of the current protocol, based on PK analysis following supraphysiological L-T(4) dosing in euthyroid volunteers, and recent recalls due to intrabrand manufacturing problems also suggest need for further refinement. We examine these interrelated issues quantitatively, using simulated what-if scenarios testing efficacy of a TSH-based protocol and tablet stability and absorption, to enhance precision of L-T(4) bioequivalence methods.

METHODS

We use an updated simulation model of human thyroid hormone regulation quantified and validated from data that span a wide range of normal and abnormal thyroid system function. Bioequivalence: We explored a TSH-based protocol, using normal replacement dosing in simulated thyroidectomized patients, switching brands after 8 weeks of full replacement dosing. We simulated effects of tablet potency differences and intestinal absorption differences on predicted plasma TSH, T(4), and triiodothyronine (T(3)) dynamics. Stability: We simulated effects of potency decay and lot-by-lot differences in realistic scenarios, using actual tablet potency data spanning 2 years, comparing the recently reduced 95-105% FDA-approved potency range with the original 90-110% range.

RESULTS

A simulated decrease as small as 10-15% in L-T(4) or its absorption generated TSH concentrations outside the bioequivalence target range (0.5-2.5 mU/L TSH), whereas T(3) and T(4) plasma levels were maintained normal. For a 25% reduction, steady-state TSH changed 300% (from 1.5 to 6 mU/L) compared with <25% for both T(4) and T(3) (both within their reference ranges). Stability: TSH, T(4), and T(3) remained within normal ranges for most potency decay scenarios, but tablets of the same dose strength and brand were not bioequivalent between lots and between fresh and near-expired tablets.

CONCLUSIONS

A pharmacodynamic TSH-measurement bioequivalence protocol, using normal L-T(4) replacement dosing in athyreotic volunteers, is likely to be more sensitive and safer than current FDA Guidance based on T(4) PK. The tightened 95-105% allowable potency range for L-T(4) tablets is a significant improvement, but otherwise acceptable potency differences (whether due to potency decay or lot-by-lot inconsistencies) may be problematic for some patients, for example, those undergoing high-dose L-T(4) therapy for cancer.

A comparative pH-dissolution profile study of selected commercial levothyroxine products using inductively coupled plasma mass spectrometry

Levothyroxine (T4) is a narrow therapeutic index drug with classic bioequivalence problem between various available products. Dissolution of a drug is a crucial step in its oral absorption and bioavailability. The dissolution of T4 from three commercial solid oral dosage forms: Synthroid (SYN), generic levothyroxine sodium by Sandoz Inc. (GEN) and Tirosint (TIR) was studied using a sensitive ICP-MS assay. All the three products showed variable and pH-dependent dissolution behaviors. The absence of surfactant from the dissolution media decreased the percent T4 dissolved for all the three products by 26-95% (at 30 min). SYN dissolution showed the most pH dependency, whereas GEN and TIR showed the fastest and highest dissolution, respectively. TIR was the most consistent one, and was minimally affected by pH and/or by the presence of surfactant. Furthermore, dissolution of T4 decreased considerably with increase in the pH, which suggests a possible physical interaction in patients concurrently on T4 and gastric pH altering drugs, such as proton pump inhibitors. Variable dissolution of T4 products can, therefore, impact the oral absorption and bioavailability of T4 and may result in bioequivalence problems between various available products.

Development and validation of an inductively coupled plasma mass spectrometry method for quantification of levothyroxine in dissolution studies

A simple, sensitive and reproducible inductively coupled plasma mass spectrometry (ICP-MS) method for the direct determination of levothyroxine (T4), based on the analysis of iodide content, in aqueous media was developed. The sample preparation consisted of addition of antimony, as the internal standard, and dilution with a 0.5% ammonia solution. The analytes were quantified at m/z 126.90 and 120.90 for iodide and antimony, respectively. The assay was linear in the concentration range of 0.1-50 ng/mL for iodide and 0.3-100 ng/mL for T4. The method was precise and accurate with lower limits of quantification (LLOQs) of 0.1 ng/mL for iodide and 0.3 ng/mL for T4. The inter-day accuracy was >94% for both analytes and the coefficient of variation (%CV) was less than 5%. The method has successfully been used for dissolution studies of T4 formulations and holds immense promise as a simple, precise and sensitive analytical technique for T4 concentration determination in in vitro studies.

Potential uses of T3 in the treatment of human disease

Treatments for hypothyroidism have been available since the late 19th century, and have been continually improved by advancing our understanding of thyroid hormone pharmacology. Thyroxine (T4) monotherapy is currently the standard of care, but may leave some hypothyroid symptoms unaddressed. Triiodothyronine (T3), formed by the monodeiodination of T4, is the biologically active form of thyroid hormone based upon its ability to regulate gene expression at the nuclear level. A variety of human and animal studies have raised the question of whether T4 monotherapy is sufficient to restore tissue and organ intracellular T3 levels to normal. Furthermore, some evidence, albeit controversial, suggests that the addition of T3 (Cytomel) to T4 replacement therapy may improve patients' quality of life, psychometric performance and mood. Further developmental work is needed to refine T3 therapy in a way to enhance efficacy and lower the potential for unwanted effects.

Hypothyroidism as a risk factor for cardiovascular disease

The cardiovascular risk in patients with hypothyroidism is related to an increased risk of functional cardiovascular abnormalities and to an increased risk of atherosclerosis. The pattern of cardiovascular abnormalities is similar in subclinical and overt hypothyroidism, suggesting that a lesser degree of thyroid hormone deficiency may also affect the cardiovascular system. Hypothyroid patients, even those with subclinical hypothyroidism, have impaired endothelial function, normal/depressed systolic function, left ventricular diastolic dysfunction at rest, and systolic and diastolic dysfunction on effort, which may result in poor physical exercise capacity. There is also a tendency to increase diastolic blood pressure as a result of increased systemic vascular resistance. All these abnormalities regress with L-T4 replacement therapy. An increased risk for atherosclerosis is supported by autopsy and epidemiological studies in patients with thyroid hormone deficiency. The "traditional" risk factors are hypertension in conjunction with an atherogenic lipid profile; the latter is more often observed in patients with TSH >10 mU/L. More recently, C-reactive protein, homocysteine, increased arterial stiffness, endothelial dysfunction, and altered coagulation parameters have been recognized as risk factors for atherosclerosis in patients with thyroid hormone deficiency. This constellation of reversible cardiovascular abnormalities in patient with TSH levels <10 mU/L indicate that the benefits of treatment of mild thyroid failure with appropriate doses of L-thyroxine outweigh the risk.