Thyroid - basic

Question 1

When can the thyroid become enlarged?

Answer 1

adolescence
pregnancy
lactation
later portion of menstrual cycle

Question 2

What is the blood supply to the thyroid

Answer 2

Superior and inferior thyroid arteries. These arise from the external carotid and the subclavian respectively.

Question 3

What is the source of the superior thyroid artery

Answer 3

External carotid

Question 4

What is the source of the inferior thyroid artery

Answer 4

Subclavian artery

Question 5

What are the parathyroid glands?

Answer 5

2-3 pairs of glands found on posterolateral aspects of the thyroid gland. They make parathyroid hormone which is involved in Ca2+ metabolism, released by low plasma Ca2_.

Question 6

When is parathyroid hormone released?

Answer 6

When there is low plasma Ca2+

Question 7

When is calcitonin released

Answer 7

When there is high plasma Ca2+

Question 8

Describe the arrangement of the cells in the follicles of the thyroid

Answer 8

Follicular cells in a circle surrounding the colloid. Parafollicular cells found between these circles.

Question 9

How is thyroglobulin in the colloid used by follicular cells in the thymus?

Answer 9

Taken in by phagosomes, broken down by lysosomes and thyroid hormone is released.

Question 10

What is the purpose of the different type of cells in the thyroid?

Answer 10

Follicular cells make thyroid hormones T4 (thyroxine) and T3 (tri-iodothyronine).

Parafollicular cells make calcitonin (released in high Ca2+)

Question 11

How is the synthesis of T3 regulated?

Answer 11

The hypothalamus releases thyrotropin releasing hormone (TRH).

TRH stimulates thyrotrophs in the anterior pituitary to release thyroid stimulating hormone (TSH).

TSH then stimulates the thyroid gland to synthesise and release thyroid hormones.

The main hormone released is T4 (think of it as a prohormone), this is converted to T3 in the periphery; T3 is more potent at receptors than T4.

T3 feeds back to thyrotrophs in the ant. pituitary to decrease release of TSH, and to the hypothalamus to reduce the release of TRH.

Question 12

How are thyroid hormones formed?

Answer 12

There is an active uptake of iodine.

Thyroglobulin is synthesised and iodine is added to it by thyroid peroxidase in the presence of H2O2.

Question 13

What enzyme is involved in the synthesis of thyroid hormones?

Answer 13

Thyroid peroxidase, in the presence of H2O2

Question 14

Is T3 or T4 more protein bound in blood?

Answer 14

T4

Question 15

How do thyroid hormones interact with nuclear receptors?

Answer 15

Thyroid hormones enter the cell by diffusion/specific carrier.

T3 enters the nucleus and binds to the thyroid receptor, which dimerises with the retinoid X receptor (a nuclear receptor).

The hormone receptor complex binds to thyroid hormones response element (TRE) on DNA (slow response).

Question 16

What does the stimulation of a thyroid receptor cause?

Answer 16

Increased mRNA production
increased protein synthesis
increased Na+/K+ ATPase levels (leading to)
increased ATP turnover, O2 consumption and increase in the number of adrenoreceptors (beta especially)

Question 17

What are the effects of thyroid hormones in the body?

Answer 17

Increased O2 consumption and heat production
Increased cardiac muscle contractility + increased sensitivity to catecholamines (more beta adrenergic receptors)
maintenance of hypoxic/hypercapnic drive in respiratory centre
increased gut motility
increased erythropoiesis
increased bone turnover
increased protein turnover (decrease in muscle mass)
increased cholesterol degredation
increased metabolic turnover of hormones and drugs

Question 18

What are the symptoms of hypothyroidism?

Answer 18

Cold intolerance
weight gain
constipation
slow reflexes (movts)
hypercholesterolaemia
dry skin
hoarse voice
coarse hair and skin
periorbital puffiness
hyperprolactinaemia (rare)

Question 19

What are some of the causes of hypothyroidism?

Answer 19

Hashimoto’s thyroiditis
Radiotherapy/surgery for hyperthyroidism
failure of delivery of thyroid hormones in the foetus (cretinism)

Question 20

What is Hashimoto’s thyroiditis

Answer 20

Affects 3% of population
Autoimmune disease
Antibodies to thyroglobulin (leads to destruction of follicular cells) or thyroid peroxidase.