No abstract

Background/Aims: The newborn screening (NBS) program in Oregon, USA, collects two routine specimens in all infants. The aim of our study was to determine the incidence of permanent versus transient congenital hypothyroidism (CH) in infants detected on the first versus second screening test. Methods: Thyroid function was determined in infants after the age of 3 years diagnosed with CH and born in Oregon between 2005 and 2011. Permanent hypothyroidism was defined as a TSH rise >10 mIU/ml after the first year on treatment or a TSH rise >6 mIU/ml with temporary discontinuation of l-thyroxine after the age of 3 years. Results: Of the cases detected on the first test, 72 of 87 (83%) were permanent and 15 of 87 (17%) were transient, while of the cases detected on the second test, 5 of 22 (23%) were permanent and 17 of 22 (77%) were transient (OR 16.3, p < 0.001). There was a female preponderance detected on the first screen versus a male preponderance on the second screen. Blood spot and serum thyroid function tests at diagnosis, before treatment, were not meaningfully different between the two groups. The mean l-thyroxine dose at the age of 3 years was greater on the first screen: 61.2 versus 36.6 μg/day. Conclusions: Infants detected on the second NBS specimen have a higher incidence of transient CH.

Resistance to thyroid hormone (RTH) is a syndrome of reduced responsiveness of target tissues to thyroid hormone (TH). It is characterized by elevated TH levels (free thyroxine [fT4] and commonly free triiodothyronine [fT3]), in association with a high or normal thyrotropin (TSH) level, in the absence of intercurrent illness, drugs, or alterations of serum TH transporter proteins. In the majority of cases, the molecular basis of RTH is a mutation in the nuclear TH receptor beta (THRB) gene (1). The estimated prevalence of RTH is 1 in 40,000 (1).The clinical and biochemical manifestations of RTH are the result of impaired gene regulation by the mutant thyroid hormone receptor (TR) b, as well as interference of the mutant TRb with the function of the wild type (WT) TRb. Since the first description of the syndrome in 1967, more than 170 mutations of the THRB gene have been identified (1) but far fewer have been published. This discrepancy is the result of the requirement for labor-intensive functional testing (mutant receptor: affinity to T3, transactivation properties, interaction with transcriptional co-activators, etc.).We identified two unrelated patients with clinical and biochemical features of RTH who were found to harbor an identical THRB gene mutation (S350L). Since this mutation had not been identified in the literature, we performed in silico modeling to predict the effect of this mutation on the TRb receptor. Modeling of the S350L Mutant TRbComputer modeling of the expected effect on the structure of TRb S350L was performed with SwissPDB viewer. The program allows three-dimensional analysis of structural alignments and comparison of active sites or other relevant parts of proteins. In addition, it permits amino acid mutations to be evaluated with simple energy calculations to allow adjustments of the torsional angles of the substituted side chains. Steric collisions, H bonds, angles, and distances are calculated. PDB ID 1XZX was used as a model in the program (2). X-ray crystallography shows the position and environment of S350 in the human TRb. For example, in PDB ID: 1XZX, S350 is 15 Å from the hormone, and 35 Å from the AF2 coregulator binding site (Fig. 1A). It is found at the start of alpha helix 8 of the receptor, with its side chain pointing inward, away from solvent. The side chain of this amino acid residue forms a hydrogen bond with the main chain carbonyl oxygen of R338, presumably stabilizing the local structure.Substitution of a leucine side chain for the serine side chain introduces hydrophobic atoms to this mixed polar plus nonpolar environment. By testing all low energy conformers of the leucine side chain, with SwissPDB viewer, we learn that the least compromised conformer of the side chain positions the C delta methyl closer than 2.5 Å to both the carbonyl oxygen of R338 and the C beta methylene of L341. The model predicts that due to steric clashes, the main chain of amino acid 350 and its immediate neighbors would have to move in the order of 1 Å to accommodate leucine at position 35...