These notes summarise the thyroid, it's anatomy, the hormones it produces as how they are regulated as well as what happens when things go wrong - looking at hyperthyroidism and hypothyroidism and the common causes, symptoms, treatment and diagnosis.
Thyroid and Disease
Thyroid Anatomy:
The thyroid is found in the neck, moves upwards when you swallow. It comprises of two lobes connected
by the isthmus. It crosses anterior to the cricoid cartilage and the trachea. Histologically, the thyroid
comprises of follicles which are a monolayer of epithelial cells enclosing a colloid. Colloid is where the
thyroid hormones are stored.
The thyroid is supplied by the superior thyroid artery (branch of the external carotid artery) and the inferior
thyroid artery (branch of the subclavian artery) which supplies mainly the posterior side of the thyroid.
There are three major veins which drain the thyroid: superior, middle and inferior thyroid vein. The first two
drain into the internal jugular vein, the latter into innominate veins.
Lymphatic drainage is to the pre-tracheal and pre-laryngeal draining into the deep cervical, supraclavicular
and mediastinal lymph nodes.
Clinically Relevant: It isn’t innervated directly, but the recurrent laryngeal passes through, and loops under
the aorta and surgery around the neck can damage this nerve causing a hoarse voice.
Thyroid Hormones:
The thyroid produces tri-iodothyronine (T3) and tetraiodothyronine (T4 aka thyroxine). It also produces
calcitonin but this is secreted independently of the other hormones.
T4 makes up around 95% of the thyroid hormones released, it can be converted into T3 in the tissues. The
synthesis requires iodine and tyrosine. Each molecule of the hormone has two tyrosine molecules with
either three or four iodine molecules added. Reverse T3 is an isomer made by the body and is completely
biologically inactive. This could be the body’s way of getting rid of excess T4.
The receptors for thyroid hormones are intracellular and nuclear receptors. When the hormones interacts
with its receptor, it migrates into the nucleus and influences gene expression to increase basal metabolic
rate. You will see an increase in carb metabolism, increase in synthesis, mobilisation and degradation of
lipids and increased protein synthesis. These hormones are essential for the normal development of the
central nervous system – the myelinations of axons.
Within the target tissues, T4 is converted into T3 (80%) and reverse T3 (20%). T3 has 40x the activity of T4
but a half life of 1 day compared to 6-8 days for T4. Within the target cell, 90% of the thyroid hormone is T3.
The hormones need to travel bound to proteins, T4 and T3 are bound to thyroxine binding proteins such as
globulin, albumin and prealbumin and only a very small amount circulates free – the unbound hormone is
what assays measure.
Thyroid Hormonal Control:
Hypothalamus releases thyrotropin-releasing hormone (TRH) which stimulates the anterior pituitary gland
to release thyroid stimulating hormone (TSH), which in turn acts on the thyroid to stimulate release of T3
and T4. T3 and T4 act as part of negative feedback loop to inhibit TSH directly and indirectly by
suppressing the release of TRH. To remove thyroid hormones, T4 and T3 are conjugated and excreted in
bile by the liver.
Thyroid Function Tests:
TSH is a biomarker for thyroid function. TSH is slow to change – around 6 weeks to reflect a change in the
thyroid, therefore not always possible to tell if there’s been a change. You are also assuming normal
pituitary function. T3 and T4 levels are also measured.
Two thyroid autoantibodies that you can test for:
TPO antibody test which is thyroid peroxidase auto-antibody – which triggers destruction of thyroid
tissue.
TRAB which is a TSH receptor autoantibody which triggers thyroid hormone production by binding
TSH receptors on thyroid.
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