“C” cells (parafollicular cells), which secrete calcitonin,a hormone that
reduces blood calcium levels. Diseases of the thyroid include Hashimoto’s
thyroiditis,an autoimmune disease, in which autoantibodies to thy-
roglobulin and thyroid peroxidase (antimicrosomal antibodies) are pro-
duced. Binding of antibodies to those molecules interferes with their
uptake and function respectively. Infiltrating T cells and autoantibodies
destroy the thyroid follicular cellsresulting in hypothyroidism.In
Hashimoto’s thyroiditis autoantibodies are also produced to the thyroid-
stimulating hormone (TSH) receptor.In that case, the autoantibody rec-
ognizes an epitope which results in blocking the activity of TSH.In
contrast, in Graves’ diseaseautoantibodies are produced to the TSH
receptor,but they are long-acting thyroid stimulating (LATS) antibod-
ies.The result is unregulated activation of the receptor and overproduction
of thyroid hormones (hyperthyroidism).
Theparathyroid glandconsists primarily of chief cells that secrete
parathyroid hormone (PTH)that increases blood calcium levels by stim-
ulating ruffled border activity in osteoclasts and decreasing renal Ca++
excretion and increasing intestinal absorption.
The pineal gland contains pinealocytes that secrete melatonin and is
innervated by postganglionic sympathetic fibers. Darkness stimulates the
production of melatonin in the pineal.
Thepancreasis both an exocrine and endocrine gland. Endocrine cells
of the pancreatic islets secrete primarily insulinandglucagon,hormones
thatregulate blood sugar by lowering and increasing gluocse respec-
tively. Blood entering the islets bypasses the peripherally located glucagon-
secreting cells to reach the more centrally-located insulin-producing beta
cells.Blood leaving the beta cells contains insulin that influences glucagon
secretion from the alpha cells.Blood leaving the islets travels to the sur-
rounding exocrine pancreas and influences secretion from the acini. In type I
diabetes mellitusformerly known as insulin-dependent diabetes mellitus
(IDDM) an autoimmune reaction results in destruction of beta cellsand the
absence of insulin. Type II diabetes, formerly known as non-insulin-dependent
diabetes, is the most common form of diabetes mellitus. Type II diabetes can
occur at any age and is reaching epidemic proportions in the United States.
Type II diabetesbegins with insulin resistance,a condition in which
adipocytes, muscle cells, and hepatocytes cannotefficiently use insulin because
of a decrease in insulin receptors or defective glucose transporter function. The
beta cells are overworked and eventually lose their ability to secrete enough
High-Yield Facts 33