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)