Thyroid hormone is essential for pituitary somatotropes and lactotropes.

Mice homozygous for a disruption in the alpha-subunit essential for TSH, LH, and FSH activity (alphaGsu-/-) exhibit hypothyroidism and hypogonadism similar to that observed in TSH receptor-deficient hypothyroid mice (hyt) and GnRH-deficient hypogonadal mutants (hpg). Although the five major hormone-producing cells of the anterior pituitary are present in alphaGsu-/- mice, the relative proportions of each cell type are altered dramatically. Thyrotropes exhibit hypertrophy and hyperplasia, and somatotropes and lactotropes are underrepresented. The size and number of gonadotropes in alphaGsu mutants are not remarkable in contrast to the hypertrophy characteristic of gonadectomized animals. The reduction in lactotropes is more severe in alphaGsu mutants (13-fold relative to wild-type) than in hyt or hpg mutants (4.5- and 1.5-fold, respectively). In addition, T4 replacement therapy of alphaGsu mutants restores lactotropes to near-normal levels, illustrating the importance of T4, but not alpha-subunit, for lactotrope proliferation and function. T4 replacement is permissive for gonadotrope hypertrophy in alphaGsu mutants, consistent with the role for T4 in the function of gonadotropes. This study reveals the importance of thyroid hormone in developing the appropriate proportions of anterior pituitary cell types.