Thyroid axis

8 important questions on Thyroid axis

What are the main components of the thyroid axis?

Hypothalamus: TRH
AP: TSH
Endocrine targets: thyroid glands, thyroid hormones (T4, T3)
Non endocrine targets: many tissues

How are thyroid hormones produced?

1. Iodide enters colloid and is oxidated to reactive iodine by thyroid peroxidase (TPO)
2. TPO catalyzes iodination of a tyrosine residue on thyroglobulins, one residue at a time
  • One iodine + one tyrosine residue = Monoiodotyrosine (MIT)
  • Two iodine + one tyrosine residue = diiodotyrosine (DIT)

3. TPO causes condensation of MIT and DIT residues
  • T3 = triiodothyronine
  • T4 = thyroxine/tetraiodothyronine

What happens in the thyroid gland when TSH stimulates it?

TSH induces follicle cells to take up thyroglobulin by endocytosis/pinocytosis, hydrolyze bound T3 and T4, and secrete hormones into the blood
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What are the effects of thyroid hormones in the organism?

- Metabolic rate regulation via increased glucose consumption and ATP production in cells
- Thermogenesis
- Increases GH and glucocorticoids   
- Cardiac function
- Lipolysis
- Bone and muscle growth

How can we diagnose euthyroid sick syndrome?

Changes to serum levels of T3 or rT3 (Reverse T3) that are not associated with a defective thyroid

What can cause euthyroid sick syndrome?

- Serious illnesses and drugs that decrease T3 / increase rT3
- Altered transport of T4
- Altered conversion of T4 to T3 due to impaired deiodinase

What are the steps to diagnosing thyroid disorders?

1. Behaviour history and weight changes
2. CBC and thyroid hormone panel (total T4, T3, free T4, T3, TSH, presence of thyroglobulin auto-antibodies)
3. Imaging/biopsy of thyroid tissue
4. TRH/TSH stimulation test to differentiate between primary and secondary
5. Differentials: ESS, adrenal hyperfunction

How can we treat thyroid disorders?

- Hypothyroidism: Levothyroxine for life
- Hyperthyroidism: radioactive iodine (I131), thyroidectomy, methimazole with rechecks, nutritional changes (iodine deficient diets)

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