Although compensated hypothyroidism (CH) is the most common thyroid impairment in Down syndrome (DS), its pathogenesis remains elusive. Because primary gonada l failure is another DS-associated endocrinopathy,we hypothesized that an impai red signal-transduction pathway shared by several organs may provide a unifying explanation for both endocrinopathies. We assessed two possible transduction-p athway components associated with CH in DS: the G-protein adenylate-cyclase (A C) system and β-adrenergic responsiveness, previously reported to be enhanced in DS fibroblasts. Twenty-one DS patients and 14 control subjects were studied. Peripheralmononuclear cells (PMCs)were incubated with G-proteinmodulators [pro stag-landin E1 (PGE1) and cholera toxin (CTx)], an AC stimulator (forskolin), and a β-adrenergic agonist (isoproterenol), and cAMP levels were determined. All pa rticipants had normal plasma thyroid hormone levels, but 11 of the DS patients h ad elevated TSH levels (hTSH), whereas in the 10 others, they were normal (nTSH) . cAMP levels in response to forskolin, PGE1, and CTx were similar in all groups , whereas isoprotereriol-stimulated cAMP levels were significantly higher in th e hTSH group than in the nTSH group and control subjects (45 ±30 versus 22 ±9 and 21 ±9 pmol ·106 cells-1 ·10 min-1, respectively; p = 0.02). Four patien ts in the DS hTSH subgroup had impaired sexual development. We found hyperrespon siveness of PMCs to a β-adrenergic agonist in a subgroup of DS patients with C H. If this observation is applicable to the thyroid gland, then it may reflect a mechanismin which negative effects on cell growth or responsiveness to TSH lead to CH.
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