4.3: The thyroid gland Flashcards by Aleksandra Kubinska

What glands are embedded in the thyroid

Parathyroid glands

How well did you know this?

Not at all

Perfectly

What nerve, supplying the vocal cords, runs close to the thyroid gland

Left recurrent laryngeal nerve

How well did you know this?

Not at all

Perfectly

Embryology of the thyroid gland

Midline outpouching from floor of pharynx
Development of thyroglobulin duct
Divided into 2 lobes
Duct disappears leaving for amen caecum
Final position by week 7
Thyroid gland then develops

How well did you know this?

Not at all

Perfectly

5 congenital anomalies of the thyroid gland

Thyroid agenesis
Lingular thyroid
Retrosternal goitre
Thyroglossal cyst
Thyroglossal fistula

How well did you know this?

Not at all

Perfectly

Role of the isthmus

Separates 2 lobes of thyroid

How well did you know this?

Not at all

Perfectly

What is the pyramid of the thyroid

3rd lobe of the thyroid, found in 10-30% of population

How well did you know this?

Not at all

Perfectly

Role of parafollicular cells

Secrete calcotonin used to produce calcium

How well did you know this?

Not at all

Perfectly

What 2 glands are important in calcium metabolism

Superior and inferior parathyroid gland

How well did you know this?

Not at all

Perfectly

What can happen if the superior or inferior parathyroid glands are damaged during surgery

Permanent under active hypoparathyroid glands - hypoparathyroidism

How well did you know this?

Not at all

Perfectly

Where is the thyroid hormone made

Colloid

How well did you know this?

Not at all

Perfectly

How does the thyroid hormone enter the circulation

Via capillaries surrounding the follicles

How well did you know this?

Not at all

Perfectly

How is thyroid hormone made within the thyroid gland

1) TSH from the blood binds to the TSH-R receptor
2) this activates the Na+/I- transporter on the basolateral membrane, stimulating active transport of iodide ions into the follicular cell
3) I- within colloid, oxidases in iodisation to form iodine
4) TSH binding also activates production of thyroglobulin
5) activates thryoperoxidase (enzyme)within cell which enters the colloid with H2O2 and catalyses iodisation reactions
6) once thyroglobulin is iodinated MIT and DIT (mono and id -iodothyrosine) are produced
7)MIT AND DIT join together to form T3 and T4
8) T3 T4 in colloid bound to thyroglobulin,
9) transported out of follicular cell by lysosome
10) T3 T4 enter circulation

How well did you know this?

Not at all

Perfectly

What is thyroglobulin and where is it produced

Prohormone -glycoprotein which can be iodinated
Produced in the nucleus of the follicular cell

How well did you know this?

Not at all

Perfectly

T3 is

Active form of thyroid hormone
Provides almost all thyroid hormone activity in target cells

How well did you know this?

Not at all

Perfectly

T4 is

Tetraiodothyronine
Prohormone converted by deiodinase enzyme into more active triiodothyronine (T3)

How well did you know this?

Not at all

Perfectly

What is the main hormone product of the thyroid gland

Thyroxine (T4)

How well did you know this?

Not at all

Perfectly

What happens to T4 in target tissues

Deiodinated to triiodothyronine (T3) - bio active form
Deiodinated in different position to produce reverse T3 (inactive)

How well did you know this?

Not at all

Perfectly

Proportions of circulating T3

80% from deiodination of T4
20% from direct thyroidal secretion

How well did you know this?

Not at all

Perfectly

Half life of T4

7-9 days

How well did you know this?

Not at all

Perfectly

Half life of T3

2 days

How are thyroid hormones transported in the blood

Bound to plasma proteins
70-80% thyroid-binding globulin TBG
10-15% albumin
prealbumin - aka transthyretin
Only 0.05% T4 and 0.5% T3 is unbound

How do T3 and T4 affect gene expression

1) enters target tissue cells through respective receptors on cell membrane
2)T4 deiodinated into active T3
3) binds to thyroid hormone receptor - nuclear receptor
4) layers gene expression

Actions of thyroid hormone in early life

Fetal growth and development - CNS
Untreated congenital hypothyroidism: cretinism

How is TSH measured in new borns

Heel-prick test

Where do foetuses gain their thyroid hormone from whilst in utero

Mothers, via placenta

4 Actions of thyroid hormone in humans

Effects on growth, CNS, GI Reproductive systems Increases basal metabolic rate Protein, carb and fat metabolism Potentiate actions of catecholamines

Control of thyroid hormone production

1) TRH released from hypothalamus to activate TSH in anterior pituitary gland 2) TSH into circulation and to thyroid gland, stimulates production of T3 and T4 3)Negative feedback at anterior pituitary level and hypothalamic level 4) Somatostatin (prod. In hypothalamus) negatively acts on anterior pituitary 5)large quantities of iodine can inhibit T3/4 production in thyroid

What can be used to urgently control thyroid levels

Potassium iodide

What happens during primary hypothyroidism

Thyroxine levels decline TSH levels climb

2 commonest forms of autoimmune thyroid disease

Hashimoto’s thyroiditis Graves’ disease Presence of one autoimmune disease increases risk of others

7 symptoms and signs of hypothyroidism

Deepening voice Depression and tiredness Cold intolerance Weight gain with reduced appetite Constipation Bradycardia Eventual myxoedema coma

What drug is used to treat hypothyroidism

Levothyroxine

Proposed mechanism of levothyroxine

Same as thyroid action T4 -> active T3

Indications of levothyroxine use

Hypothyroidism Hyperthyroidism- blocking and replacement regimen

Dosing of levothyroxine

Adjusted according to TSH - aim to get normal range Common dose 100 micro grams Most commonly administered orally once daily

Potential minor complications of levothyroxine

Weight loss Headache

Potential major complications of levothyroxine

Heart attack Rapid HR

Alternative therapy for hypothyroidism

Combined thyroid hormone replacement

Benefits of combined thyroid hormone replacement

Combination of T3 and T4 - some improvement in wellbeing

Disadvantages of combined thyroid hormone replacement

T3 has a shorter half life, so needs to be administered multiple times a day Complicated symptoms of “toxicity” - palpitations, tremor, anxiety Often surpresses TSH

In hyperthyroidism

Thyroid makes too much thyroxine Thyroxin levels rise TSH levels drop

3 causes of hyperthyroidism

Graves’ disease - whole gland smoothly enlarged and whole gland overactive Toxic multinodular goitre Solitary toxic nodule

What kind of disease is Graves’ disease

Autoimmune disease

In Graves’ disease

Antibodies bind to and stimulate TSH receptor in thyroid Other antibodies bind to muscles behind eye Or antibodies stimulate growth of soft tissue of shins

In Graves’ disease the binding to and stimulation of TSH receptors in thyroid is called

Smooth goitre

In Graves’ disease binding of antibodies to muscles behind the eye is called

Exophthalamos

In Graves’ disease the stimulation of growth of soft tissue in the shins due to antibodies is called

Pretibial myxoedema

8 symptoms and signs of hyperthyroidism

Heat intolerance Weight loss and increased appetite Myopathy Mood swings Diarrhoea Tremor of hands Palpitations Sore eyes, goitre