The suffix -tropin indicates "an affinity for". Thyrotropin has an affinity for the thyroid. Thyrotropin is known also as thyroid stimulating hormone (TSH).
Thyrotropin (or, if you prefer, TSH) promotes the growth of the thyroid gland in the neck and stimulates it to produce more thyroid hormones. When there is an excessive amount of thyroid hormones, the pituitary gland stops producing TSH, reducing thyroid hormone production. This mechanism maintains a relatively constant level of thyroid hormones circulating in the blood.