Endocrinology – thyroid

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Endocrinology – thyroid
Rudolf Cardinal, 18 Oct 98
The thyroid
The thyroid is butterfly-shaped, having two lateral lobes and a central pyramidal lobe. Very vascular. Thyroid tissue
contains follicles which are surrounded by cuboidal epithelium – the follicular cells. The follicular cells make the principal thyroid hormone, thyroxine. Scattered in and around the follicles are parafollicular or clear (C) cells. The C
cells make calcitonin (see handout on calcium). Don’t confuse the thyroid with the parathyroid glands (which make
PTH, see handout on calcium).
Embryology. The thyroid derives from the pharynx, which is why remnants can turn up in the tongue (a lingual thyroid). It is therefore embryologically exocrine. It migrates down to lie in front of the trachea, just below the cricoid cartilage. The C cells have a different origin in neural crest tissue.
Thyroid hormones – thyroxine (T4) and triiodothyronine (T3)
Nature and synthesis
• Thyroid hormones contain iodine. Terrestrial vertebrates don’t get much, so the thyroid is adapted to trap it.
• Iodide is trapped and transported into follicular cells, then excreted into the lumen of the follicle. The cell also
makes thyroglobulin (TG) and exocytoses this into the lumen. The iodide (I–) is oxidised by a peroxidase on the
luminal surface of the cell. The resulting free iodine (I) reacts non-enzymatically with tyrosine residues of the thyroglobulin in the follicle, a process called iodination. This forms mono- and diiodotyrosine residues (MIT, DIT),
which are coupled together, possibly by the same peroxidase, to form mainly thyroxine (T4) and some 3,5,3’triiodothyronine (T3). This process may be catalysed by the peroxidase, but is certainly helped by the structure of
the thyroglobulin molecule.
• The follicle contains a colloid of thyroglobulin with MIT, DIT, T4 and T3 on it. There are no cells within the follicle.
• The follicles may be used to store thyroid hormones. Next, the colloid is pinocytosed back into the cell and the thyroglobulin is split off by proteolysis to leave the free hormone in vesicles that may be exocytosed into the circulation. MIT and DIT that is taken up is recycled within the cell.
• Thyroid hormones are water-insoluble, like steroids, so they require specific carrier/binding proteins in the plasma
(and in the cell cytosol, to gain access to the nucleus). Thyroxine binds to thyroxine-binding prealbumin (TBPA)
and thyroxine-binding globulin (TBG), especially the latter, which carries 75% of all T4 and T3. It also binds to
albumin; though the affinity is very low, there’s so much albumin around that it carries a fair amount of thyroid
hormone. Only about 0.5% of the serum T4 and 0.3% of the T3 is in the free state, but this is the physiologically active form (see below). Half-lives: T3 about 1.3 days, T4 about 7 days.
Mode of action
• Thyroid hormones act on nuclear T3/T4 receptors to regulate gene transcription. (Subsequent effects include activation of the Na+/K+SXPSLQFUHDVHGPLWRFKRQGULDOUHVSLUDWLRQDQGSRVVLEOHLQFUHDVHG DGUHQRFHSWRUV\QWKHVLV1.)
• T3 versus T4. Much more T4 is secreted than T3 (ratio ~30:1). However, most T4 is monodeiodinated to T3 in the
periphery. In fact, about 85% ends up as T3. T3 is the major physiologically active thyroid hormone, but T4 may
have important developmental roles.
• Reverse T3 (rT3). This is a structural isomer of T3 that is inactive as a hormone – see “Control”, below.
Functions
• Not essential for life. Was originally thought so, but those operations had removed the parathyroid (q.v.).
• Thyroid hormones have a general stimulating effect on metabolism. They affect nearly every cell in the body.
They promote thermogenesis, via the actions on the sodium pump and mitochondrial activity. An optimal level promotes protein synthesis (i.e. deficiency/excess impairs it). All aspects of lipid metabolism are stimulated. Carbohydrate metabolism is also stimulated: intestinal absorption and glucose uptake by adipose tissue are stimulated (an effect that potentiates insulin), while the hyperglycaemic/glycogenolytic effects of adrenaline are potentiated and insulin degradation is increased (reducing insulin sensitivity).
• Growth and development. Required for normal maturation of bone and CNS, for fetal lung surfactant production
and for milk secretion. See also section on disease, below.
• Permissive actions. Required for the actions of other hormones on target tissues, probably because thyroid hormones induce the synthesis of proteins that the other hormones act on.
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hyperthyroidism.
1
Control
•
•
•
•
TSH from the pituitary stimulates thyroid hormone secretion via a plasma membrane receptor on the follicular cell.
TRH from the hypothalamus stimulates TSH release.
Negative feedback. Thyroid hormones inhibit both hypothalamic TRH and pituitary TSH secretion.2
Tissue regulation. During fasting, peripheral tissues make less T3 and more rT3. So the less glucose is available, the
less T3 is created and the less the cell’s metabolism is stimulated. High carbohydrate intake increases T3 levels,
which increases diet-induced thermogenesis. Low-carbohydrate diets have the opposite effect. This could be an adaptation to allow the organism to eat enough to obtain scarce nutrients (e.g. protein) without being burdened with a
weight gain.
• Stress inhibits TSH secretion. In many mammals, including infant but perhaps not adult humans, exposure to cold
causes rapid TSH release.
• Learn and Remember – it’s the free hormone level that matters. For example, TBG levels double in pregnancy, so
free T4 levels initially fall, but the feedback systems ensure increased T4 synthesis as a result. The system will equilibriate around a new level with almost double the total T4 present, but the same free concentration as before. This
applies to many hormones and other protein-bound electrolytes that you will come across, and it is vital that you
understand this principle.
Disease – syndromes
• Deficiency in adults hypothyroidism, a.k.a. myxoedema (myxa mucus, oidema swelling – a mucinous protein is
deposited subcutaneously because mucopolysaccharide metabolism impaired). Sluggish, slow reflexes, ‘peaches and
cream’ complexion, sensitive to cold, low basal metabolic rate, decreased cardiac output and heart rate, impaired
intestinal glucose absorption, high plasma cholesterol, overweight. In women, menorrhagia and impaired fertility.
• Deficiency in children cretinism. This has all the features of adult hypothyroidism, but also severe growth retardation and intellectual impairment. (Bone elongation and maturation is impaired – GH secretion is diminished; thyroid hormones are required for the action of GH; other effects as well. CNS maturation impaired.) Characteristic appearance: facial bone development retarded, but tongue still grows so it appears too large for the mouth.
• Excess in adults hyperthyroidism, a.k.a. thyrotoxicosis. Restless, irritable, thin, high BMR, increased catabolism, high CO/HR, low cholesterol, etc. A severe hyperthyroid crisis (“thyroid storm”) is dangerous – cardiac arrhythmias.
Disease – iodine
• Iodine deficiency causes thyroid hypertrophy and goitre. 3 Iodine deficiency GHFUHDVHGWK\URLGKRUPRQHV\QWKesis LQFUHDVHG76+ IROOLFXODUFHOOK\SHUWURSK\DQGK\SHUSODVLD
• Certain drugs can inhibit iodide transport (e.g. thiocyanates). After a while, this causes thyroid hypertrophy and
goitre. Some plants contain cyanogenic glucosides (apricots, cherries, almonds, cassava, sweet potato) or thioglucosides (the Brassica family – cabbages, Brussels sprouts, cauliflowers etc.), both of which are metabolised to thiocyanates. These plants are therefore dietary goitrogens, and may be responsible for endemic goitre in parts of the
world (esp. where cassava is a staple food). Other drugs can inhibit different synthetic steps. Even iodine is transiently inhibitory to thyroid function in large doses – mechanism unknown.
Disease – neoplasia and autoimmunity
Many, but note the following:
• Hypothyroidism may be due to thyroid disease (primary hypothyroidism) or pituitary disease (secondary). How
would you distinguish the two?
• Tumours in the thyroid may be inactive or active; if they are active they may cause hyperthyroidism.
• Graves’ disease is an autoimmune disease, in which antibodies are made to the TSH receptor. The antibodies have
the property of mimicking the action of TSH, so stimulate the thyroid and cause hyperthyroidism. The exophthalmos
(protrusion of the eyes) is not a feature of the hyperthyroidism, but of an autoimmune-mediated infiltration behind
the eye. See a picture and you’ll never forget.
2
T4 has more hypothalamic action and T3 inhibits the pituitary.
Once known as “Derbyshire neck” – this area was historically deficient in iodine. Cretinism due to iodine deficiency is
still a major problem in parts of the world.
3

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