O efeito molecular e estrutural do hormônio tiroideano no esqueleto

Resistance to thyroid hormone due to mutations in the THRB gene impairs bone mass and affects calcium and phosphorus homeostasis

CONTEXT

Resistance to thyroid hormone (RTH) is an inherited syndrome of reduced tissue responsiveness to thyroid hormone, which is usually due to mutations in the thyroid hormone receptor β gene (THRB). Few studies have been conducted to investigate bone and mineral metabolism in RTH.

OBJECTIVE

The objective of the study was to evaluate the clinical and biochemical parameters related to bone and mineral metabolism in RTH due to mutations in the THRB gene (RTHβ).

DESIGN AND PARTICIPANTS

We conducted a cross-sectional study on 14 patients with RTHβ (RTHG), eight adults and six children, and 24 control subjects (CG).

OUTCOMES

Serum measures included total calcium (TCa), inorganic phosphate (iP), alkaline phosphatase (AP), parathyroid hormone (PTH), 25-hydroxyvitamin D (25OHD), osteocalcin (OC), carboxyterminal telopeptide (CTX), and fibroblast growth factor 23 (FGF-23). We estimated the renal threshold phosphate concentration (TmPO4/GFR) and assessed bone mass using dual X-ray absorptiometry.

RESULTS

Adults and children with RTH showed higher serum levels of TCa than controls (P=.029 and, P=.018 respectively). However, only children with RTH exhibited lower serum levels of iP than controls (P=.048). FGF-23 was higher in RTHβ children (P=.04). RTHβ adults had lower whole-body (P=.01) and lumbar spine (P=.01) bone mineral density than control subjects. The same pattern was observed when the results were expressed as Z-scores between groups, with a lower value in RTHG than in CG for the lumbar spine of adults (P=.03). No difference was observed between groups in PTH, 25OHD, AP, OC, and CTX.

CONCLUSION

Biochemical abnormalities are seen in children with RTH (Low iP, high FGF23), while high calcium (with normal UCa) is seen in RTH subjects of all ages, and later on, in adult life, low BMD is seen. Considering that the TRα1 isoform is the predominant TR in the skeleton, we hypothesize that probably these patients may exhibit enhanced calcium flux from bone to circulation. Our data represent a challenge for new studies to unveil the control of calcium and phosphorus homeostasis and fracture risk in these patients.

The multiple effects of thyroid disorders on bone and mineral metabolism

Differently from most hormones, which commonly are specialized molecules able to influence other cells, tissues and systems, thyroid hormones (TH) are pleiotropic peptides, whose primordial function is difficult to identify. The complex action of TH on human economy can be easily witnessed by examining the diverse consequences of TH excess and deficiency during development and after maturity. In particular, different manifestations in bone modeling and remodeling reflect the circumstantial consequences of thyroid disturbances, which are age dependent. While hyperthyroidism during childhood enhances bone mineralization and accelerates epiphyseal maturation, in adults it induces bone loss by predominant activation of osteoclast activity. Furthermore, the syndrome of TH resistance is a multifaceted condition in which different sites exhibit signs of hormone excess or deficiency depending on the configuration of the TH receptor isoform. The investigation of the impact of TH resistance on the skeleton still remains to be elucidated. We present here a thorough review of the action of TH on bone and of the impact of thyroid disorders, including hyper- and hypothyroidism and the syndrome of TH resistance, on the skeleton.

Atraso na erupção dental em adolescente com hipotireoidismo

OBJETIVO: Relatar o caso de adolescente que procurou serviço odontológico de um ambulatório de medicina do adolescente por apresentar atraso de erupção dentária. A atuação multiprofissional permitiu chegar ao diagnóstico de hipotireoidismo. DESCRIÇÃO DO CASO: Paciente de 16 anos e 4 meses cuja queixa inicial foi atraso na erupção dentária. Na avaliação odontológica, chamava atenção o importante atraso na erupção dentária, o que poderia estar relacionado a alguma moléstia orgânica. Desta forma, solicitou-se a intervenção da equipe médica, que, no acompanhamento, constatou se tratar de hipotireoidismo. COMENTÁRIOS: Chama-se a atenção o fato de que adolescentes prezam a aceitação pelos seus pares e as questões estéticas e, muitas vezes, a procura por um serviço de saúde envolve queixas que podem parecer triviais. Porém, o olhar atento não deixará que problemas orgânicos mais graves passem despercebidos. Salienta-se ainda a importância da atuação da equipe multiprofissional.