The first case of association between postpartum thyroiditis and thyroid hormone resistance in an Italian patient showing a novel p.V283A THRB mutation

Thyroid hormone resistance: Mechanisms and therapeutic development

As an essential primary hormone, thyroid hormone (TH) is indispensable for human growth, development and metabolism. Impairment of TH function in several aspects, including TH synthesis, activation, transportation and receptor-dependent transactivation, can eventually lead to thyroid hormone resistance syndrome (RTH). RTH is a rare syndrome that manifests as a reduced target cell response to TH signaling. The majority of RTH cases are related to thyroid hormone receptor β (TRβ) mutations, and only a few RTH cases are associated with thyroid hormone receptor α (TRα) mutations or other causes. Patients with RTH suffer from goiter, mental retardation, short stature and bradycardia or tachycardia. To date, approximately 170 mutated TRβ variants and more than 20 mutated TRα variants at the amino acid level have been reported in RTH patients. In addition to these mutated proteins, some TR isoforms can also reduce TH function by competing with primary TRs for TRE and RXR binding. Fortunately, different treatments for RTH have been explored with structure-activity relationship (SAR) studies and drug design, and among these treatments. With thyromimetic potency but biochemical properties that differ from those of primary TH (T3 and T4), these TH analogs can bypass specific defective transporters or reactive mutant TRs. However, these compounds must be carefully applied to avoid over activating TRα, which is associated with more severe heart impairment. The structural mechanisms of mutation-induced RTH in the TR ligand-binding domain are summarized in this review. Furthermore, strategies to overcome this resistance for therapeutic development are also discussed.

Mutational Landscape of Resistance to Thyroid Hormone Beta (RTHβ)

Resistance to thyroid hormone beta (RTHβ) is a syndrome characterized by reduced responsiveness of peripheral tissues to thyroid hormone (TH). In most cases, the disorder is associated with germline pathogenic variants in the thyroid hormone receptor beta (THRB) gene. This paper summarizes the clinical and biochemical presentation of the disease, providing a comprehensive overview on molecular genetic features. Particular care is given in reporting all identified THRB variants with an assessed or unknown clinical significance. Our aim is to offer a useful tool for clinical and genetic specialists in order to ease clinical diagnosis and genetic counseling.

Resistance to thyroid hormone β in autoimmune thyroid disease: a case report and review of literature

Background

Resistance to thyroid hormone beta (RTHβ) results in symptoms of both increased and decreased thyroid hormone action. The effect of thyroid hormone changes in different types of autoimmune thyroid disease (AITD) in RTHβ is dynamic.

Case presentation

A 25-year-old Asian female had a RTHβ Y321C mutation with Hashimoto’s thyroiditis and type 2 diabetes mellitus. She was followed-up through gestation and two years postpartum, revealing development of postpartum thyroiditis (PPT) with characteristic wide fluctuations in serum thyrotropin levels, and of spontaneous recovery from an episode of transient hypothyroidism. The presence of RTHβ did not prolong thyroiditis duration nor progressed toward permanent hypothyroidism. Prenatal genetic analysis was not performed on the unaffected fetus, and did not result in congenital hypothyroidism, possibly because maternal free thyroxine (FT4) levels were mildly elevated at less than 50% above the reference range in early gestation and gradually decreased to less than 20% after the 28th gestational week.

Conclusion

In RTHβ patients with autoimmune thyroid disease, episodes of thyroid dysfunction can significantly alter thyrotropin levels. During pregnancy, mildly elevated maternal free thyroxine levels less than 20% above the upper limit may not be harmful to unaffected fetuses. Unnecessary thyroid hormone control and fetal genetic testing was avoided during the gestational period with monthly follow-up.

Prenatal Diagnosis of Resistance to Thyroid Hormone and Its Clinical Implications

CONTEXT

Resistance to thyroid hormone-β (RTH-β) is an autosomal dominant disorder characterized by reduced sensitivity of target tissues to thyroid hormones (THs). Individuals with RTH-β have high TH levels usually due to mutations in the TH receptor-β (THRB) gene. The management of RTH-β during pregnancy is challenging, as wild-type (WT) fetuses born to RTH-β mothers have low birth weight and suppressed postnatal thyroid-stimulating hormone (TSH), due to intrauterine exposure to excess TH.

OBJECTIVE

To determine birth weight and postnatal TSH of WT fetuses carried by mothers with RTH-β whose fT4 levels were maintained below 20% of the upper limit of normal (ULN).

DESIGN

Retrospective chart review.

SETTING

Academic institution in collaboration with off-site hospitals and private practices.

PATIENTS

Thirteen women harboring THRB gene mutations were evaluated during 18 pregnancies.

INTERVENTION

Prenatal genetic diagnosis by amniocentesis. Women carrying WT fetuses were given the option of treatment with antithyroid medication by their treating physicians with the aim to avoid serum fT4 levels above 20% of the ULN.

RESULTS

No significant difference was found in birth weight corrected for gestational age and in serum TSH levels at birth between WT and RTH-β infants born to RTH-β mothers.

CONCLUSIONS

Prenatal diagnosis may play an important role in the management of RTH-β during pregnancy. Aiming for maternal fT4 levels not above 50% of the ULN in RTH-β mothers carrying WT fetuses seems to be a prudent approach that prevents the otherwise expected low birth weight and postnatal TSH suppression.

Autoimmune Abnormalities of Postpartum Thyroid Diseases

The year following parturition is a critical time for the de novo appearance or exacerbation of autoimmune diseases, including autoimmune thyroid disease. The vast majority of postpartum thyroid disease consists of postpartum thyroiditis (PPT) and the minority by Graves' disease and non-autoimmune thyroiditis. PPT has a worldwide prevalence ranging from 1 to 22% and averaging 5% based on a review published in 2012. Several factors confer risk for the development of PPT. Typically, the clinical course of PPT is characterized by three phases: thyrotoxic, hypothyroid, and euthyroid phase. Approximately half of PPT women will have permanent hypothyroidism. The best humoral marker for predictivity, already during the first trimester of gestation, is considered positivity for thyroperoxidase autoantibodies (TPOAb), though only one-third to half of such TPOAb-positive pregnant women will develop PPT. Nutraceuticals (such as selenium) or omega-3-fatty acid supplements seem to have a role in prevention of PPT. In a recent study on pregnant women with stable dietary habits, we found that the fish consumers had lower rates of positivity (and lower serum levels) of both TPOAb and thyroglobulin Ab compared to meat eaters. Finally, we remind the reader of other diseases that can be observed in the postpartum period, either autoimmune or non-autoimmune, thyroid or non-thyroid.

Conservative management of pregnancy in patients with resistance to thyroid hormone associated with Hashimoto's thyroiditis

BACKGROUND

Resistance to thyroid hormone (RTH) is a rare thyroid disorder characterized by elevated free thyroid hormones with non-suppressed thyrotropin (TSH). Guidelines for the management of pregnancy in patients with RTH are not well defined. Chorionic villus biopsy is sometimes proposed to manage treatment based on the genotype of the fetus.

PATIENT FINDINGS

A woman with RTH (c1243C>T, pR320C mutation in the thyroid hormone receptor β (THRB gene)) associated with Hashimoto's thyroiditis (HT) had three successful pregnancies. During the pregnancies, the mother was treated with levothyroxine (LT4). She never underwent chorionic villus sampling. The babies had normal birth weights. The first child harbored the THRB mutation.

SUMMARY AND CONCLUSIONS

The management of pregnancies in patients with RTH and the indication for chorionic villus sampling are discussed in these cases. In RTH patients, pregnancies must be planned and closely followed. There is no need for prenatal diagnosis of RTH if the patient, due to limited thyroidal reserve, cannot produce excess of thyroid hormones to harm a normal fetus. The more common challenge in RTH remains how to best manage high maternal thyroid hormone levels, and whether or not to lower thyroid hormone levels based on the genotype of the fetus.