Effect of phenytoin therapy on thyroid function

From hormone replacement therapy to regenerative scaffolds: A review of current and novel primary hypothyroidism therapeutics

Primary hypothyroidism severely impacts the quality of life of patients through a decrease in the production of the thyroid hormones T3 and T4, leading to symptoms affecting cardiovascular, neurological, cognitive, and metabolic function. The incidence rate of primary hypothyroidism is expected to increase in the near future, partially due to increasing survival of patients that have undergone radiotherapy for head and neck cancer, which induces this disease in over half of those treated. The current standard of care encompasses thyroid hormone replacement therapy, traditionally in the form of synthetic T4. However, there is mounting evidence that this is unable to restore thyroid hormone signaling in all tissues due to often persistent symptoms. Additional complications are also present in the form of dosage difficulties, extensive drug interactions and poor patience compliance. The alternative therapeutic approach employed in the past is combination therapy, which consists of administration of both T3 and T4, either synthetic or in the form of desiccated thyroid extract. Here, issues are present regarding the lack of regulation concerning formulation and lack of data regarding safety and efficacy of these treatment methods. Tissue engineering and regenerative medicine have been applied in conjunction with each other to restore function of various tissues. Recently, these techniques have been adapted for thyroid tissue, primarily through the fabrication of regenerative scaffolds. Those currently under investigation are composed of either biopolymers or native decellularized extracellular matrix (dECM) in conjunction with either primary thyrocytes or stem cells which have undergone directed thyroid differentiation. Multiple of these scaffolds have successfully restored an athyroid phenotype in vivo. However, further work is needed until clinical translation can be achieved. This is proposed in the form of exploration and combination of materials used to fabricate these scaffolds, the addition of peptides which can aid restoration of tissue homeostasis and additional in vivo experimentation providing data on safety and efficacy of these implants.

Effects of Hepatic Drug-metabolizing Enzyme Induction on Clinical Pathology Parameters in Animals and Man

Hepatic drug-metabolizing enzyme (DME) induction is an adaptive response associated with changes in preclinical species; this response can include increases in liver weight, hepatocellular hyperplasia and hypertrophy, and upregulated tissue expression of DMEs. Effects of DME induction on clinical pathology markers of hepatobiliary injury and function in animals as well as humans are not well established. This component of a multipart review of the comparative pathology of xenobiotically mediated induction of hepatic metabolizing enzymes reviews pertinent data from retrospective and prospective preclinical and clinical studies. Particular attention is given to studies with confirmation of DME induction and concurrent evaluation of liver and/or serum hepatobiliary marker enzyme activities and histopathology. These results collectively indicate that in the rat, when histologic findings are limited to hepatocellular hypertrophy, DME induction is not expected to be associated with consistent or substantive changes in serum or plasma activity of hepatobiliary marker enzymes such as alanine aminotransferase, alkaline phosphatase, and gamma glutamyltransferase. In the dog and the monkey, published studies also do not demonstrate a consistent relationship across DME-inducing agents and changes in these clinical pathology parameters. However, increased liver alkaline phosphatase or gamma glutamyltransferase activity in dogs treated with phenobarbital or corticosteroids suggests that direct or indirect induction of select hepatobiliary injury markers can occur both in the absence of liver injury and independently of induction of DME activity. Although correlations between tissue and serum levels of these hepatobiliary markers are limited and inconsistent, increases in serum/plasma activities that are substantial or involve changes in other markers generally reflect hepatobiliary insult rather than DME induction. Extrahepatic effects, including disruption of the hypothalamic-pituitary-thyroid axis, can also occur as a direct outcome of hepatic DME induction in humans and animals. Importantly, hepatic DME induction and associated changes in preclinical species are not necessarily predictive of the occurrence, magnitude, or enzyme induction profile in humans.

Thyroglobulin and thyroid hormones in patients on long-term treatment with phenytoin, carbamazepine, and valproic acid

Serum concentrations of total triiodothyronine (T3) and thyroxine (T4) as well as serum thyroid-stimulating hormone (TSH) and thyroglobulin (Tg) were measured in 24 patients with epilepsy taking anticonvulsants (either phenytoin, carbamazepine, or valproic acid as single treatment) and in a control group of 28 patients with scoliosis but without thyroid disease. The T4 as well as the TSH concentrations were depressed in patients on phenytoin or carbamazepine treatment. The T3 concentration was increased in the patients on carbamazepine or valproic acid treatment, whereas the Tg levels were unaffected by all three drugs. Thus, a slight depression of the TSH concentration within the normal range does not influence the Tg release. The lack of change in the Tg concentration also speaks against a direct effect of the antiepileptic drugs on the thyroid gland.

Increased frequency of goitre in epileptic patients on long-term phenytoin or carbamazepine treatment

Thyroid function, the clinical occurrence of goitre and ultrasonically determined thyroid gland volume were investigated in 23 patients with phenytoin- and 28 patients with carbamazepine-treated convulsive disorders and compared with matched healthy controls. In the phenytoin treated group median thyroid volume was 26 ml (range 14-57 ml) compared to 17 ml (range 8-41 ml) in the controls (P less than 0.01). Ten patients and four controls had a goitre (NS). Median serum T4 and FT4I levels were reduced, serum TSH level increased and serum T3, T3RU, FT3I and thyroglobulin levels unaltered compared with the controls. In the carbamazepine treated group median thyroid volume was 25 ml (range 13-66 ml) compared to 16 ml (range 9-44 ml) in the controls (P less than 0.01). Thirteen patients and three controls had a goitre (P less than 0.02). Median serum T4, FT4I and FT3I levels were reduced, serum thyroglobulin increased and serum T3, T3RU and TSH levels unaltered compared with the controls. The increase in thyroid size is probably a compensatory mechanism due to the low free thyroid hormones in serum caused by an increased hepatic degradation of thyroid hormones by phenytoin and carbamazepine.

Avian muscular dystrophy: serum thyroid defect and limited improvement with methimazole and propylthiouracil

Serum concentrations of triiodothyronine (T3) and thyroxine (T4) were determined by radioimmunoassay in normal and genetically related muscular dystrophic chicks at 2 through 42 days ex ovo. There were no significant differences in T4 concentrations, but T3 concentrations were reduced about 35% below normal values in dystrophic birds at 14 to 42 days. The situation was reversed, however, on day 2, with T3 concentrations about 50% greater in dystrophic than in normal serum. Administration of T3 beginning on day 2 ex ovo did not alter phenotypic expression of dystrophic signs. Administration of the thyroid "antagonists," methimazole and propylthiouracil, however, significantly increased righting ability and reduced serum creatine kinase activity in dystrophic chicks. None of the administered substances improved the histopathology of dystrophic pectoralis major muscles. The data indicate that serum T3 concentrations may provide an early "marker" for avian dystrophy, and suggest that lowered serum T3 concentrations in older chicks may represent a compensatory response to the elevated serum T3 in newly hatched dystrophic chicks.

Anticonvulsant drugs. An update

A considerable amount of information is now available concerning the clinical pharmacology of the anticonvulsant drugs. Some of the more important data are reviewed in this article. In recent years, valproic acid (or sodium valproate) has found a place as a major anticonvulsant agent, while older drugs such as troxidone and sulthiame seem to be disappearing from use. Although much information is available, the essential mechanisms of action of the anticonvulsant drugs are still not understood, either at a molecular or at an electrophysiological level. The pharmacokinetics of the anticonvulsants in common use are now reasonably well documented, though some minor questions are still to be answered. Numerous interactions between anticonvulsants and endogenous substances or other drugs administered concurrently (including other anticonvulsants) have been recorded, but much work still needs to be done to elucidate the frequency and mechanisms of the various interactions. Many adverse effects of the anticonvulsants are known, but further unwanted effects of long-established drugs continue to emerge from time to time, including the still somewhat controversial matter of anticonvulsant-related dysmorphogenesis. The use of valproic acid and its sodium salt has been associated with a worrying incidence of serious liver and pancreatic toxicity. Adequate basic data are now available to put the clinical use of anticonvulsants on a rational basis, but much work remains to be done in this area. In particular, the question of 'therapeutic ranges' of plasma concentrations of the various drugs needs to be reinvestigated in a rigorous statistical fashion, and in relation to different clinical types of epilepsy. The usefulness of monitoring free rather than total drug concentrations also needs further investigation. The ultimate test of the validity of all background scientific pharmacological information about anticonvulsants is its usefulness in the treatment of patients with epilepsy.

Side effects of phenobarbital and phenytoin during long-term treatment of epilepsy

Phenobarbital and phenytoin have good antiepileptic effect, but clinically significant untoward effects occur during their long-term use. Phenobarbital may cause hyperactivity, behavioral problems, sedation, and even dementia; these effects are dose related to some extent. Side effects of phenytoin include sedation, a cerebellar syndrome, phenytoin encephalopathy, psychosis, locomotor dysfunction, hyperkinesia, megaloblastic anemia, decreased serum folate level, decreased bone mineral content, liver disease, IgA deficiency, gingival hyperplasia, and a lupus-like hypersensitivity syndrome. Especially susceptible to the neurotoxic effects of phenytoin are epileptic children with severe brain damage who are on multiple drugs. In those children, balance disturbance may develop and be followed by gradual loss of locomotion. Among 131 mentally retarded epileptic patients, phenytoin intoxication occurred in 73 (56%), of whom 18 experienced persistent loss of locomotion. There is experimental evidence that the toxic action of phenytoin lies at the cellular level, predominantly in the cerebellum. Many experts avoid the long-term use of phenytoin because of its insidious and potentially dangerous side effects.

Serum thyroid hormones and blood folic acid during monotherapy with carbamazepine or valproate. A controlled study

Studies investigating the influence of antiepileptic drugs on thyroid hormones usually have compared patients chronically treated with antiepileptic drugs to controls. To date, this type of designs has produced divergent results both with regard to individual drugs and individual thyroid hormones. The present study comprised 31 patients with newly diagnosed epilepsy, commencing treatment with either carbamazepine or valproate. T3, T4, FT4, FT3, rT3, TSH, T3 resin uptake and blood folic acid, were determined before and during antiepileptic monotherapy, thus making the patient his own control. During treatment with carbamazepine, a significant decrease in T4, FT4, FT3, rT3 and TBG was observed. Valproate caused a decrease in T4, FT4 and T3. Neither of the drugs caused any changes in blood folic acid concentrations or persistent increases in the TSH values. None of the patients developed overt symptoms of hypothyreoidism. Conceivable mechanisms underlying these hormonal changes are reviewed.