93- Why is short stature a common manifestation of juvenile hypothyroidism?
Short stature with retarded bone age is a common manifestation of juvenile
hypothyroidism. This is because thyroid hormone is required for GH secretion
and GH-mediated IGF1 generation. In addition, thyroid hormone has a direct
effect on growth plate and promotes differentiation of chondrocytes to hyper-
trophic chondrocytes and enhances angiogenesis.- What are the unique features of precocious puberty associated with primary
hypothyroidism?
The characteristic features of precocious puberty associated with primary
hypothyroidism are decreased growth velocity and retarded bone age, whereas
other causes of precocious puberty are associated with increased growth veloc-
ity and advanced bone age. Girls with precocity due to primary hypothyroid-
ism usually present with vaginal bleed and/or thelarche, whereas boys present
with isolated testicular enlargement. Pubic/axillary hair is characteristically
absent in both sexes despite precocity. This is because precocious puberty
associated with hypothyroidism is predominantly mediated through FSH, and
lack of LH drive results in decreased androgen production. In addition, thyroid
hormone plays an important role in the growth and development of piloseba-
ceous unit. - Why do patients with juvenile hypothyroidism develop precocious puberty?
Precocious puberty associated with primary hypothyroidism is gonadotropin-
independent. Basal FSH is elevated; however, basal LH is low, and LH response
to GnRH is prepubertal. Precocious puberty in patients with primary hypothy-
roidism is due to the action of TRH and TSH on GnRH and FSH receptors,
respectively (“specifi city-spillover”). The loss of feedback inhibition of T 4 on
hypothalamus results in elevated levels of TRH, which acts on GnRH receptors
and preferentially stimulates FSH secretion. LH secretion is suppressed as a
result of hyperprolactinemia, which is a consequence of elevated TRH, whereas
FSH secretion is driven by elevated TRH, even in presence of hyperprolac-
tinemia. In addition, the clearance of FSH is also reduced as a consequence of
hypothyroidism. As both TSH and FSH are glycoprotein hormones, elevated
levels of TSH in patients with primary hypothyroidism act on FSH receptors in
ovary and testes, to induce ovarian follicular growth and testicular enlargement,
respectively.- What are the causes of stippled epiphysis?
Stippled epiphysis is a feature of congenital/juvenile hypothyroidism, thyroid
hormone resistance, and multiple epiphyseal dysplasias. Thyroid hormone is
essential for the development of growth plate; therefore, thyroid hormone defi -
ciency/resistance results in fragmentation of ossifi cation centers leading to
stippled epiphysis (Fig. 3.15 ).3 Thyroid Disorders in Children