The process of producing thyroid hormones is a complex multistep one, and thyroid-stimulating hormone (TSH) plays a significant role in stimulating various aspects of thyroid function. Additionally, TSH has a strong tropic effect, promoting the growth, multiplication, and survival of thyroid epithelial cells. In regions with limited iodide availability, which is essential for thyroid hormone synthesis, TSH levels are elevated due to reduced negative feedback. Elevated TSH levels can lead to the enlargement of the thyroid gland, resulting in a noticeable bulge in the neck known as a goiter.

The thyrotrope in the pituitary gland is stimulated by thyrotropin-releasing hormone (TRH), which is produced by a specific subset of parvocellular cells. TRH can be found in Table 41.1. 

Factors regulating the secretion of corticotropin-releasing hormone (CRH), which affects the hypothalamic-pituitary-adrenal axis, are illustrated in Figure 41.11. These factors include physical, diurnal rhythms, emotional stimuli, and others, all of which influence the secretion of adrenocorticotropin hormone (ACTH). Corticotrophs, which produce ACTH, are subject to long feedback (cortisol) and short feedback (ACTH). Chemical factors such as hypoglycemia also play a role in this axis.

Figure 41.12 displays pituitary glycoprotein hormones. Human chorionic gonadotropin (hCG) is produced by the placenta and binds to the luteinizing hormone (LH) receptor. Follicle-stimulating hormone (FSH) and LH are other glycoprotein hormones, and thyroid-stimulating hormone (TSH) is the hormone of interest here. Each hormone has its own respective subunit, and they interact with their corresponding receptors. The alpha-glycoprotein subunit (α-GSU) is also present.
Chapter 42, production of thyroid hormones is a complex multistep process, and TSH stimulates essentially every aspect of thyroid function. TSH also has a strong tropic effect and stimulates hypertrophy, hyperplasia, and survival of thyroid epithelial cells. In geographical regions where the availability of iodide is limited (iodide is required for the synthesis of thyroid hormone), TSH levels are elevated because of reduced negative feedback. Elevated TSH levels can produce striking growth of the thyroid, producing a bulge in the neck called a goiter.
•Fig. 41.10 DiurnalpatternofserumACTH.(ModifiedfromPor-terfieldSP,WhiteBA.Endocrine Physiology. 3rded.Philadelphia:Mosby;2007.)Awake Sleep Serum ACTH level
The pituitary thyrotrope is stimulated by the releasing hormone thyrotropin-releasing hormone (TRH) (see
Table 41.1). TRH, produced by a subset of parvocellular •Fig. 41.11 Hypothalamic-pituitary-adrenalaxisillustratingfactorsregulatingsecretionofcorticotropin-releasinghormone(CRH).ACTH,adrenocorticotropichormone.(ModifiedfromPorterfieldSP,WhiteBA.Endocrine Physiology. 3rded.Philadelphia:Mosby;2007.)CRH Physical Diurnal rhythms Emotional Others Set point ACTH Corticotroph Long feedback (cortisol) Short feedback (ACTH) Chemical (hypoglycemia) •Fig. 41.12 Pituitaryglycoproteinhormones.hCGismadebytheplacenta(see )andbindstotheLHreceptor.FSH,follicle-stimulatinghormone;hCG,humanchorionicgonadotropin;LH,luteinizinghormone;TSH,thyroid-stimulatinghormone.˜-TSH subunit ˜-FSH subunit ˜-LH subunit ˜-hCG subunit TSH FSH LH hCG TSH receptor FSH receptor LH receptor LH receptor Plus °-glycoprotein subunit (°-GSU)