Endocrinology (thyroid)

Question 1

Describe the thyroid gland anatomy

Answer 1

The thyroid glands comprise a pair of separate elongated lobes lying lateral to the trachea immediately below the larynx and adjacent to the first five or six tracheal rings

Question 2

Is there any thyroid tissue in other locations

Answer 2

Accessory and ectoptic thyroid tissue is not uncommon and may be located at various site (e.g., sublingual) but is most commonly found in the thorax (e.g., in the periaortic fat)

Question 3

What is the link between the thyroid glands and the parathyroid glands

Answer 3

The thyroid glands are intimately associated with the parathyroid glands. There are two pairs of parathyroid glands (the internal ones towards tha caudal pole and the external ones at the cranial pole)

Question 4

Explain briefly the thyroid gland physiology

Answer 4

The uptake of iodine and release of thyroid hormones (T3 and T4) by follicular cells is under the influence of thyroid stimulating hormone (TSH) secreted by the pituitary adenohypophysis

Secretion of TSH is inhibited by circulating T4 which reduces the sensitivity of the pituitary to thyrotropin-releasing hormone (TRH) derived from the hypothalamus

Question 5

Describe the mechanism of thyroid hormones production

Answer 5

1/ Ingested iodine is converted to iodide in the GI tract where it is absorbed
2/ Thyrocytes actively trap iodide from blood via a iodide pump
3/ Iodide undergoes oxidation to iodine
4/ Iodine combines with tyrosine amino acid residues on thyroglobulin molecules and form mono- and di-iodotyrosines (MIT and DIT)
5/ MIT and DIT can couple to form tri-iodothyronine (T3) and thyroxine (T4)
6/ The thyroglobulin undergoes proteolysis within the thyrocyte to release MIT, DIT, T3 and T4
7/ A proportion of T4 is de-iodinated to form T3 and rT3 before its release
8/ Following cleavage of T4 and T3 from the thyroglobulin molecule in the follicular cells, these hormones are secreted into the circulation
Approximately 95% of hormone secretion is T4, the remainder is T3
T3 is about four times more potent than T4 and approximately 50% of T3 is derived from extra-thyroidal de-iodinationation of T4 (circulating T4 levels are thus a more accurate indicator of thyroid function than T3)
9/ In the blood, only around 0.1% of T4 and 1% of T3 are free and thus availaible to tissues. The remainder is bound to plasma proteins

Question 6

How is the thyroid hormone secretion regulated

Answer 6

1/ TRH (thyrotropin releasing hormone)
- produced in the hypothalamus and transported to the anterior pituitary where it stimulates TSH secretion

2/ TSH (thyroid stimulating hormone)
- Produced by anterior pituitary
- stimulates iodide uptake by thyroid tissue
- stimulate production and secretion of thyroide hormones
- may have an inhibitory effect on TRH release

3/ T4 and T3
- produced and released by the thyroid gland
- inhibit TSH (and possibily TRH) production and secretion