INTRODUCTION

The thyroid gland weighs 10 to 20 grams in normal adults. The normal thyroid gland is immediately below the larynx and encircles the front and side portion of the trachea.

The parathyroid glands are in close approximation with the thyroid gland and function independent of the thyroid gland. Normal parathyroid glands are approximately the size of a grain of rice or a lentil. Normal glands are usually about 5 by 4 by 2 millimeters in size and weigh 35 to 50 milligrams.

FUNCTION

Thyroid hormones, thyroxine (T4), and 3,5,3′-triiodothyronine (T3), are critical determinants of brain and somatic development in infants and of metabolic activity in adults; they also affect the function of virtually every organ system. Thyroid hormone biosynthesis and secretion are maintained within narrow limits by a regulatory mechanism that is sensitive to small changes in circulating hormone concentrations.

The parathyroid glands produce parathyroid hormone (PTH) which is one of the two major hormones modulating calcium and phosphate homeostasis; the other hormone is calcitriol (1,25-dihydroxyvitamin D). PTH also stimulates the conversion of calcidiol (25-hydroxyvitamin D) to calcitriol in renal tubular cells, thereby stimulating intestinal calcium absorption.