Clinical Consequences of Variable Results in the Measurement of Free Thyroid Hormones: Unusual Presentation of a Family with a Novel Variant in the THRB Gene Causing Resistance to Thyroid Hormone Syndrome

Unusual causes of hyperthyrotropinemia and differential diagnosis of primary hypothyroidism: a revised diagnostic flowchart

The clinical consequences of primary hypothyroidism include cardiovascular morbidity, increased mortality, and poor quality of life; therefore guidelines endorsed by several Scientific Societies recommend measuring circulating thyroid-stimulating hormone (TSH) in patients at risk. The assessment of serum TSH levels is also deemed to be the most robust and accurate biomarker during the management of replacement therapy in patients with a previous diagnosis of primary hypothyroidism. In line with a reflex TSH laboratory strategy, free thyroxine is measured only if the TSH falls outside specific cutoffs, in order to streamline investigations and save unjustified costs. This serum TSH-based approach to both diagnosis and monitoring has been widely accepted by several national and local health services; nevertheless, false-negative or -positive testing may occur, leading to inappropriate management or treatment. This review aims to describe several infrequent causes of increased circulating TSH, including analytical interferences, resistance to TSH, consumptive hypothyroidism, and refractoriness to levothyroxine replacement treatment. We propose a clinical flowchart to aid correct recognition of these various conditions, which represent important potential pitfalls in the diagnosis and treatment of primary hypothyroidism.

Expanding the phenotype of THRB: a range of macular dystrophies as the major clinical manifestations in patients with a dominant splicing variant

Inherited retinal dystrophies (IRDs) are a clinically and genetically heterogeneous group of disorders that often severely impair vision. Some patients manifest poor central vision as the first symptom due to cone-dysfunction, which is consistent with cone dystrophy (COD), Stargardt disease (STGD), or macular dystrophy (MD) among others. Here, we aimed to identify the genetic cause of autosomal dominant COD in one family. WGS was performed in 3 affected and 1 unaffected individual using the TruSeq Nano DNA library kit and the NovaSeq 6,000 platform (Illumina). Data analysis identified a novel spliceogenic variant (c.283 + 1G>A) in the thyroid hormone receptor beta gene (THRB) as the candidate disease-associated variant. Further genetic analysis revealed the presence of the same heterozygous variant segregating in two additional unrelated dominant pedigrees including 9 affected individuals with a diagnosis of COD (1), STGD (4), MD (3) and unclear phenotype (1). THRB has been previously reported as a causal gene for autosomal dominant and recessive thyroid hormone resistance syndrome beta (RTHβ); however, none of the IRD patients exhibited RTHβ. Genotype-phenotype correlations showed that RTHβ can be caused by both truncating and missense variants, which are mainly located at the 3' (C-terminal/ligand-binding) region, which is common to both THRB isoforms (TRβ1 and TRβ2). In contrast, the c.283 + 1G>A variant is predicted to disrupt a splice site in the 5'-region of the gene that encodes the N-terminal domain of the TRβ1 isoform protein, leaving the TRβ2 isoform intact, which would explain the phenotypic variability observed between RTHβ and IRD patients. Interestingly, although monochromacy or cone response alterations have already been described in a few RTHβ patients, herein we report the first genetic association between a pathogenic variant in THRB and non-syndromic IRDs. We thereby expand the phenotype of THRB pathogenic variants including COD, STGD, or MD as the main clinical manifestation, which also reflects the extraordinary complexity of retinal functions mediated by the different THRB isoforms.