How are prolactin and thyroid‑stimulating hormone (TSH) related?

Relationship Between Prolactin and TSH

Prolactin and TSH are functionally related through shared hypothalamic regulation, co-secretion patterns in pituitary adenomas, and bidirectional influences in thyroid dysfunction states.

Shared Hypothalamic-Pituitary Regulation

Co-pulsatile Secretion

• Both TSH and prolactin exhibit significant nonrandom co-pulsatile secretion patterns, with 36-45% of TSH pulses occurring within 15 minutes of prolactin pulses, suggesting a common central regulatory mechanism 1
• This co-pulsatile release occurs independently of TRH stimulation, as constant TRH infusion does not alter the pulse concordance rates 1
• The underlying pulse generator remains unidentified but demonstrates coordinated hypothalamic control of both hormones 1

TRH-Mediated Interactions

• TRH stimulates release of both TSH and prolactin from the anterior pituitary 2
• In hyperprolactinemic states, hypothalamic dopamine exerts inhibitory effects on both prolactin and TSH responses to TRH stimulation 3
• Baseline TSH levels are significantly suppressed in hyperprolactinemic patients compared to normoprolactinemic individuals, likely through dopaminergic mechanisms 3

Co-secretion in Pituitary Adenomas

Growth Hormone-Secreting Adenomas

• TSH can be co-secreted by somatotrophinomas, though less frequently than prolactin co-secretion 2
• In children and adolescents with GH excess, 65% demonstrate hyperprolactinemia at presentation, while TSH co-secretion occurs but is less common 2
• Dynamic pituitary assessment should evaluate for both hypofunction and hyperfunction of all anterior pituitary hormones, including TSH and prolactin 2

Clinical Implications for Adenoma Evaluation

• Age-dependent and sex-dependent IGF1 evaluation should always accompany prolactin assessment in children and adolescents with prolactinomas to rule out mixed hormone hypersecretion 2
• Baseline and dynamic pituitary assessment can identify excess or deficiency of other anterior pituitary hormones when hyperprolactinemia is present 2

Thyroid Dysfunction and Prolactin

Primary Hypothyroidism

• Severe primary hypothyroidism causes hyperprolactinemia in 43% of women and 40% of men with frank hypothyroidism, likely due to compensatory TRH hypersecretion 2
• Subclinical hypothyroidism is associated with hyperprolactinemia in 36% of women and 32% of men 2
• Pituitary hyperplasia from chronic TRH stimulation in hypothyroidism must be distinguished from true prolactinoma 2
• Hypothyroid patients show exaggerated prolactin responses to TRH (mean maximum increment 100.5 ng/ml), which normalize with thyroid hormone replacement (falling to 36.1 ng/ml) 4

Hyperthyroidism

• Hyperthyroid patients demonstrate blunted prolactin responses to TRH (mean maximum increment 14.2 ng/ml), which increase to normal levels (46.9 ng/ml) after achieving euthyroidism 4
• Thyroid hormone levels inversely correlate with TRH-induced prolactin release: subnormal thyroid hormones increase prolactin release, while supranormal levels inhibit it 4

Dissociation Patterns

• In treated thyrotoxicosis, TSH and prolactin response patterns frequently dissociate, with blunted TSH responses commonly persisting alongside normal or exaggerated prolactin responses 5
• This dissociation indicates that while both hormones share regulatory pathways, they can respond independently to thyroid status changes 5

Clinical Assessment Priorities

When Evaluating Hyperprolactinemia

• Always measure TSH to exclude primary hypothyroidism as the cause of elevated prolactin 2
• Severe and prolonged primary hypothyroidism can disrupt kidney and liver function while delaying growth and puberty, compounding the clinical picture 2
• Consider that hypothyroidism-related pituitary enlargement may mimic prolactinoma on imaging 2

When Evaluating Pituitary Masses

• Screen for both prolactin and TSH abnormalities, as co-secretion or stalk effect can affect both hormones 2
• Stalk compression from mass lesions disrupts dopaminergic inhibition, potentially elevating both prolactin and affecting TSH regulation 2

Monitoring Considerations

• Regular monitoring of thyroid function tests (TSH, free T4, free T3) is essential when managing prolactin disorders, as the bidirectional relationship affects treatment outcomes 6
• Prolactin may serve as a more sensitive early indicator of thyroid insufficiency than TSH or thyroxine levels alone in some pituitary disorders 7