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Published on: March 14, 2017
Inhibited hypothalamic-pituitary-thyroid axis in type I pseudohypoparathyroidism
1Veterans Affairs Medical Center, Des Moines, Iowa, and the University of Iowa School of Medicine, Iowa City, Iowa.
Summary
Patients with pseudohypoparathyroidism show inhibited thyroid function. This may be due to impaired cyclic adenosine monophosphate (cAMP) signaling, affecting thyroid-stimulating hormone (TSH) and thyrotropin-releasing hormone (TRH) responses.
Area of Science:
- Endocrinology
- Molecular Endocrinology
Background:
- Pseudohypoparathyroidism (PHP) is a rare genetic disorder characterized by resistance to parathyroid hormone (PTH).
- Patients typically present with hypocalcemia, hyperphosphatemia, and elevated PTH levels.
- The hypothalamic-pituitary-thyroid (HPT) axis function in PHP is not fully understood.
Purpose of the Study:
- To evaluate the hypothalamic-pituitary-thyroid axis in two patients diagnosed with type I pseudohypoparathyroidism.
- To investigate the impact of pseudohypoparathyroidism on the response to thyroid-stimulating hormone (TSH) and thyrotropin-releasing hormone (TRH).
Main Methods:
- Assessed serum levels of thyroxine (T4) and triiodothyronine (T3), T3 resin uptake, and 24-hour thyroidal uptake of radioactive iodine (123I).
- Measured responses to subcutaneous TSH administration and intravenous TRH stimulation.
- Compared results with 10 healthy control subjects.
Main Results:
- Thyroid hormone levels and iodine uptake were within normal ranges in patients with pseudohypoparathyroidism.
- Patients exhibited significantly blunted responses to both TSH and TRH compared to controls.
- Basal TSH levels were elevated, and TRH-induced TSH responses were blunted relative to basal concentrations.
Conclusions:
- Pituitary thyrotroph response to TRH and thyroid gland response to TSH are significantly inhibited in pseudohypoparathyroidism.
- This inhibition may stem from inadequate cyclic adenosine monophosphate (cAMP) generation, a crucial second messenger for hormone action.
- Impaired cAMP signaling could underlie the observed HPT axis dysfunction in this syndrome.
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