Thyroid Disorder

1. PHMD 446: THYROID AND
ANTITHYROID DRUGS
1

7. REVIEW OF THYROID PHYSIOLOGY

10. The thyroid gland consists of folicular cells
• TSH binds to receptor on the follicular cell and sends a
signal to nucleus of the cell
• The signal increases expression of gene that codes for
synthesis of a protein – Thyroglobulin
• The signal also stimulates the enzyme thyroperoxidase –
TPO
• Iodine is oxidised by TPO. Oxidised iodine rapidly
iodinates tyrosine residues within thyroglobulin to form
monoiodotyrosine (MIT) and diiodotyrosine (DIT). The
process is called iodide organification.

11. Thyroidal peroxidase (TPO) is transiently blocked by high levels of intrathyroidal
iodine and blocked more persistently by thioamide drugs
• Two molecules of DIT combine in thyroglobulin to form L-thyroxine (T4)
• One MIT + DIT form T3
• Thyroxine, T3, MIT, DIT are released from thyroglobulin by exocytosis and
proteolysis of thyroglobulin. The MIT and DIT are de-iodinated within the
gland and the iodine reutilised.
• The process of proteolysis is also blocked by high levels of intrathyroidal
iodide.
• The ratio of T4:T3 is approx. 5:1. So most of the hormone released is
thyroxine (T4).
• Most of T3 circulating is from peripheral conversion of T4 to T3.

12. T4 crosses into tissues and is converted to active T3
by the enzyme de-iodinase.
T3 acts on genes that increases expression of many
enzymes. One such enzyme is Na/K ATPase
This enzyme uses a lot of ATP. This requires
metabolism – glycolysis, lipolysis, glycogenolysis.
Thus increase metabolism generating heat/body
temperature

15. FIGURE 38–3 The hypothalamic-pituitary-thyroid axis. Acute psychosis or prolonged exposure to cold may activate the axis.
Hypothalamic thyroid-releasing hormone (TRH) stimulates pituitary thyroid-stimulating hormone (TSH) release, while somatostatin and
dopamine inhibit it. TSH stimulates T4 and T3 synthesis and release from the thyroid, and they in turn inhibit both TRH and TSH
synthesis and release. Small amounts of iodide are necessary for hormone production, but large amounts inhibit T3 and T4 production and
release. Solid arrows, stimulatory influence; dashed arrows, inhibitory influence. H, hypothalamus; AP, anterior pituitary.
B. Autoregulation of the Thyroid Gland
The thyroid gland also regulates its uptake of iodide and thyroid hormone synthesis by intrathyroidal mechanisms that are independent of

16. TRANSPORT OF THYROID HORMONES
• Thyroxine and T3 in plasma reversibly bind to proein –
thyroxine binding globulin (TBG)T3).
• Many physiologic states and drugs affect T4, T3 and
thyroid transport.
• Drugs such as amiodarone, Lithium, iodinated contrast
media, B-blockers, and corticosteroids and sevre illness
or starvation inhibit conversion of T4 to T3

17. Common effects of Thyroid hormone:
• Heart: activates B1 – increases HR, stroke volume increasing
CO
• Bone: maintains balance between osteoblasts and osteoclast
• Nervous system: increase symthatetic drive
• GIT: increase motility and secretions
• Skin: increase cutaneous blood flow nourshing hair, skin and
nail growth. Also acts on sebaceous glands
• Reproductive system: devlopment of reproductive system,
regulate sex hormone binding globulin levels
• Muscle tissue: regenration and growth of skeletal muscles

27. ANTI- THYROID AGENTS
• The thioamides - methimazole,
propylthiouracil
• Carbimazole which is converted to
• Methimazole is 10x more potent than
propylthiouracil
• Due to warnings about hepatitis
propylthiouracil is reserved for use in first
trimester of preganancy, in thyroid storm and in
patients intolerant to methimazole.

29. Distinguish between primary
and secondary hyperthyroidism
β-HCG produced by
choriocarcinoma in
uterus. They act like TSH
stimulating thyoid gland.
Since pituitary is normal,
TSH level will reduce but
T4, T3 will remain high

30. Produce TSH receptor
anti-bodies which
stimulates the gland to
produce T4, T3

31. PATHOPHYSIOLOGY OF TOXIC ADENOMA AND
TOXIC MULTINODULAR
The nodules are caused by TSH receptor mutations
which are capable of stimulating TSH receptors
(auto-stimulation)

34. Thyroid acropathy – digital clubbing

39. Sudden acute exacerbation
of all symptoms of
thyrotoxicosis

66. NEXT LECTURE

81. This is an end stage of untreated hypothyroidism.
All drugs are admin. IV BECAUSE these pts absorb drugs
poorly from other routes
These pts have parge pools of empty T3 and T4 binding
sites which must be filled before there is adequate free
thyroxine to affect metabolism. A loading dose of L-
thyroxine is therefore given initially.

82. Hypothyroidism and pregnancy
• Hypothyroid women are often anovulatory and therefore
infertile until restoration of euthyroid state.
• This has led to widespread use of thyroid hormone for
fertility. But no evidence of usefulness in euthyroid pts
• In pregnant hypothyroid women, its essential to ensure
daily dose of thyroxine is adequate BECAUSE development
of fetal brain depends on maternal thyroxine.

83. Drug-induced hypothyroidism
• Can be satisfactorily managed with levothyroxine if the
offending agent cannot be stopped
• With amiodarone-induced hypothyroidism, L-thyroxine
wmay be required after stopping amiodarone BECAUSE of
its long half life