Both hyper- and hypothyroidism may result in menstrual disturbances. In hyperthyroidism, amenorrhea was described as early as 1840 by von Basedow. The most common manifestation is simple oligomenorrhea (decreased menstrual flow). Anovulatory cycles are very common. Increased bleeding may occur, but is rare in hyperthyroidism. Nowadays hyperthyroidism is diagnosed earlier than it once was, and so the clinical picture is generally milder. So, menstrual disorders are less common than in previous series. In a recent paper, 21.5% of 214 patients had disturbances in their cycle, compared to 50% in some older series. In hypothyroidism, on the contrary, polymenorrhea (increased menstrual bleeding) is more common. Defects in hemostasis may contribute to this. Anovulation may be represent. Fertility is reduced in both hyper- and hypothyroidism, and the outcome of pregnancy is more often abnormal than in euthyroid women. It is of interest that in juvenile hypothyroidism precocious puberty has been described. This is probably due to a "spillover" effect of the glucoprotein hormones: TSH, which is markedly increased in hypothyroidism, has a small FSH- and LH-like effect. Galactorrhea may also be present in hypothyroidism, possibly because TSH, the hypophyseal TSH-releasing hormone, increases the secretion of both TSH and PRL.