Enhanced prolactin secretion in patients with primary hypothyroidism during thyroid replacement

Area of Science:

• Endocrinology
• Neuroendocrinology

Background:

• Thyroid hormones modulate prolactin (PRL) secretion indirectly via hypothalamic dopamine and directly at the pituitary.
• Primary hypothyroidism is associated with altered pituitary hormone regulation.

Purpose of the Study:

• To investigate the impact of thyroid hormones on hypothalamic dopamine neurons, pituitary lactotrophs, and thyrotrophs in primary hypothyroidism.
• To examine changes in basal PRL, TSH, and PRL response to dopamine blockade during thyroid hormone replacement therapy.

Main Methods:

• Administered increasing doses of thyroid hormones to patients with primary hypothyroidism.
• Monitored basal PRL, TSH, and PRL response to sulpiride (dopamine receptor blocker).
• Assessed hormonal changes in relation to thyroid hormone levels and euthyroid state.

Main Results:

• Hyperprolactinemia was observed in 10/24 hypothyroid patients.
• A significant negative correlation existed between plasma T3/T4 and basal TSH.
• Thyroid hormone administration led to paradoxical transient PRL increases and steady TSH decrease.
• PRL response to sulpiride became exaggerated during treatment and normalized upon achieving euthyroid state.

Conclusions:

• Thyroid hormones may stimulate hypothalamic dopaminergic activity and lactotroph activity in a long-term hypothyroid state.
• Initial low-dose thyroid hormone activation of lactotrophs might overcome dopaminergic inhibition, causing paradoxical PRL elevation.