The effects of prolactin and TSH on thyroid function in Fundulus heteroclitus

Treatment with ovine prolactin significantly reduces serum T₄ in Fundulus heteroclitus, both in control (P < 0.05) and TSH-stimulated fish (P < 0.01), without affecting thyroidal radioiodine uptake. Prolactin also reduces the loss of radioiodine from F. heteroclitus (P < 0.001). This loss is markedly increased by ovine TSH (P < 0.001), but only in the absence of prolactin. Although other explanations are possible, prolactin is probably goiterogenic in F. heteroclitus, lowering thyroidal secretion of T₄ as well as inhibiting the peripheral effects of this hormone.