Case 9 SBA Flashcards
Describe the structure of the thyroid gland
A butterfly shaped endocrine gland. Has a left and right lobe connected by a central isthmus. Anterior to and wrapping partially around the trachea. Surrounded by the pretracheal fascia. Not visible or palpable unless pathology is present
Describe the parathyroid glands
usually two pairs of small glands, one superior and one inferior in the left and right lobes. Some people, however, have more pairs. Share a similar blood supply and lymphatic drainage to the thyroid gland. Produces parathyroid hormone in response to low blood calcium.
Embryology of thyroid
descends through thyroglossal duct which then closes over time. If it does not close, a thyroglossal cyst can form which is a differential diagnosis for a midline neck lump
Perfusion of thyroid gland
superior thyroid artery (external carotid) and inferior thyroid artery (thyrocervical trunk from subclavian artery)
Drainage of thyroid gland
superior, middle, and inferior (superior and middle to internal jugular and inferior to brachiocephalic)
Innervation of thyroid gland
sympathetic: fibres from superior, middle, inferior cervical ganglia. parasympathetic: fibres from superior and recurrent laryngeal nerves
Lymph node groups that drain thyroid gland
peri-thyroid, pre-laryngeal, pre-tracheal, paratracheal, superior and inferior deep cervical
Histology of thyroid gland
functional units of the thyroid are the follicles. A single layer of follicle cells surrounds a pool of colloid. The production and storage of thyroid hormones occurs in the colloid. C cells secrete calcitonin which is involved in calcium homeostasis
TSH normal range
0.4-4.2 mlU/L.
Free T4 normal range
0.8-2.7ng/dL
Total T4 normal range
4.5-11.7 ug/dL
T3 normal range
80-220 ng/dL
Graves’ disease investigation results
serum T4 elevated (both free and total), TSH supressed, increased iodide uptake by thyroid (123I RAIU test)
Untreated hyperthyroidism investigation results
low TSH, high free T4, high T3, high radioiodine uptake
Hyperthyroidism with thyrotoxicosis investigation results
low TSH, normal free T4, high T3, normal or high radioiodine uptake
Untreated primary hypothyroidism investigation results
high TSH, low free T4, low or normal T3, low or normal radioiodine uptake
Hypothyroidism secondary to pituitary disease investigation results
low or normal TSH, low free T4, low or normal T3, low or normal radioiodine uptake
Euthyroid on exogenous thyroid hormone investigation results
normal TSH, normal free T4 on T4, low free T4 on T3, high T3 on T3, normal T3 on T4, low radioiodine uptake
Describe levothyroxine
Hypothyroid treatment of choice. T4. Consistent potency, >99.8% protein bound (gives reservoir of T4 for conversion to T3), half-life of roughly 7 days, 5-6 weeks for new steady state, 80% oral bioavailability (reduced by gastric acidity, various foods and drugs, especially Fe and Ca supplements).
Describe liothyronine
T3, hypothyroidism. Cytomel, triostat. Quicker onset, half-life >24 hours, more variable response, frequent dosing, more expensive, increased cardiovascular events
Describe liotrix
hypothyroidism. T4/T3 combo in 4:1 ratio, brand name thyrolar. Attempts to simulate natural hormone levels, no therapeutic advantage over T4, expensive
Describe desiccated animal thyroid
hypothyroidism. 4:1 ratio of T4:T3, variable response, interchange with other therapies is problematic, allergic reactions to animal proteins. Not really used that often anymore.
Describe carbimazole
hyperthyroidism. pro-drug, meaning only active when converted in thiamazole and methimazole metabolites
T4 drug side effects (TWIST CAD)
not for treatment of obesity or for weight loss, increased risk of cardiac events in elderly (MI and A-Fib), adrenal insufficiency, decreased bone mineral density, tachycardia, tremors, insomnia, weight loss, sweating
Drug-drug interactions of T4
long list - altered absorption, protein binding, and metabolism that may alter therapeutic response, decreased conversion to T3 by beta-blockers, amiodarone, and propylthiouracil (PTU).
Thioamide side effects
agranulocytosis, liver toxicity, and congenital malformations
High iodine side effects (MIDBAD)
metallic taste, burning mouth, diarrhoea, systemic allergic reactions (fever, rash, rhinitis), severe dermatitis, iododerma eruptions can be fatal but are very rare
Indications for T3/T4 treatment
replacement/supplemental in patients with hypothyroidism of any aetiology, treatment or prevention of euthyroid goitres, adjunct to surgery and radioiodine therapy in management of TSH-dependent thyroid cancer
Goal of hypothyroidism treatment
maintain TSH in normal range for symptom relief and avoiding hypothyroidism in foetus
Thioamide mechanism of action
thiourea blocks hormone production. Bind to active site of thyroid peroxidase (TPO), complexes with iodine and converts it to iodide, this oxidases thioamide which forms disulphide bond with enzyme, causing irreversible inactivation of TPO enzyme. Decreased formation of iodotyrosine residues causes a decreased in T4 synthesis. Inhibition of peripheral deiodination of T4.
Thioamide examples
Propylthiouracil (PTU), carbimazole, and methimazole (MMI)
Indications for carbimazole
graves’ disease and toxic multinodular goitre in patients for whom surgery and radioactive iodine are not appropriate
In preparation for surgery with 131Iodine until effective
What is favoured as hyperthyroidism treatment in 1st trimester of pregnancy and thyroid storm and why?
PTU as blocks T4-T3 conversion
Treatment of Graves’ disease in pregnancy
1st trimester: PTU
2nd and 3rd trimesters: carbimazole but continue 2-3 weeks pre-birth.
Nursing: carbimazole as PTU contraindicated.
Give in low doses and monitor TSH as both can cross placenta
High iodide treatment mechanism
acute inhibition of tyrosine iodination, then decreases in iodine uptake transporter. Inhibits proteolysis and release of T4 – rapid effect but not sustained, escape after 1-2 months
Indications for high iodide treatment
Rarely used as a sole therapy: preoperative with thioamide for thyroidectomy, with PTU for thyroid storm, prevent radiation-induced thyroid cancer after nuclear fallout
Forms of high iodide treatment
SSKI, Lugol’s solution, Iodoral
131 Iodine indications and mechanism
gland ablation in hyperthyroidism. Half-life of 8 days, gamma and beta emitter – long exposure for ablation, causes cell necrosis and destroys small blood vessels, symptoms abate in 6 months – thioamides, iodide, or beta-adrenergic antagonists given over this lag period
Treatment of thyrotoxicosis (LAB)
beta adrenergic antagonists
lithium
amiodarone
Describe thyroxine
T4, major product. Daily secretion around 100 nmoles. 5-7 day half life. 0.04% free, rest bound to plasma proteins
Describe triiodothyronine
T3. 3-8 times more active than thyroxine. Daily secretion around 5 nmoles. 0.4% free, rest bound to plasma proteins. 80% derived from peripheral metabolism of T4.
Which proportion of thyroid hormone is active?
The unbound, <1%
Purpose of the bound thyroid hormone?
Acts as a reservoir in the circulation with hormone dropping off the transport protein when required to maintain the equilibrium between free and bound
70% T3 and T4 bound to
Thyroxine-binding globulin (TBG)
10% T3 and T4 bound to
Transthyretin (thyroxine-binding prealbumin)
Ten-fold greater affinity for T4
Albumin with thyroid hormones
15-20% bound to albumin, major source of free hormone from rapid dissociation as bound with relatively low affinity
Four types of thyroid hormone receptors
TRα1, β1, β2, and β3
Which thyroid hormone receptor type has a very low affinity for T3?
TRα1
What do the TRβ1, β2, and β3 have a higher affinity for?
T3
Describe the mechanism of thyroid hormone
Once inside the cell, T4 is acted upon by deiodinase enzymes which forms T3 + Iodine. Receptors are found attached to hormone response elements in DNA where they repress transcription – causes conformational change which allows gene transcription to occur. Thyroid hormones change the amount of protein in the cell and thus the function of the cell. Key target is increased transcription of genes encoding mitochondrial uncoupling proteins.
Effects of thyroid hormones
increases basal metabolic rate, increases cardiac output, enhances catecholamine actions, maintains emotional tone, effect on foetal neural development, bone growth, tooth development, permissive role in male and female reproductive organs
Synthesis of thyroid hormone
production of thyroglobulin (glycoprotein rich in tyrosine) produced by follicular cells and exocytosed across the apical membrane to be stored in the colloid. Iodide retrieved from blood by I/Na transporter, taken into colloid by protein pendrin iodide oxidized by thyroid peroxidase (TPO, expressed on follicular cell membrane) into iodine. Iodine groups are added to tyrosine on thyroglobulin (either one or two per tyrosine) under control of TPO. Coupling – T1 and T2 coupled to form T3 and T4 groups on the thyroglobulin which is stored in the colloid.