Internal Diseases and Therapy · MU Sofia

78. Hypothyroidism

Covers the thyroid gland's anatomy and how thyroid hormones and calcitonin are made, then explains primary and secondary hypothyroidism with their causes, symptoms, investigations and levothyroxine management. It also describes myxoedema coma, congenital iodine deficiency, and Hashimoto's and atrophic thyroiditis, with a hormone synthesis diagram and a histology micrograph.

Inside: 1 section · 2 labelled pictures · 16 flashcards · 4 min

Topic 79 of 139 in the MU Sofia syllabus for Internal Diseases and Therapy (Medicine, Years 4 and 5). The free AceMed note covers it in 1 section, about 708 words (4 min of reading), with 16 flashcards.

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  1. Hypothyroidism

Hypothyroidism

All right kids, let's talk about thyroid gland! It is an endocrine gland situated anterior to the trachea with two opposing lobes joined by a central isthmus. The thyroid follicles are rich in a colloid, viscous fluid in which thyroid hormone production takes place. The hormone is produced when TSH (released by the anterior pituitary) binds to receptors on the thyroid follicle stimulating entry of iodine ions into the follicle. Oxidation and bonding of iodine ions with thyroglobulin within the colloid produces triiodothyronine (T3) and tetraiodothyronine (T4 aka thyroxine).

Image: “Thyroid hormone synthesis” by Mikael Häggström . When using this image in external works, it may be cited as: Häggström, Mikael (2014). " Medical gall, CC0, via Wikimedia Commons.

Less than 1% of T3 and T4 are unbound, the vast majority is bound to protein such as thyroid binding globulin (aka TBG) to prevent uptake. T3 is more metabolically active than T4, most T3 is produced by the enzymatic removal of iodine from T4 in the periphery, T4 is the main thing produced by the thyroid. Thyroid hormones have widespread regulatory and metabolic effects.

Calcitonin is also produced from the thyroid gland (specifically from parafollicular or C cells). Calcitonin is produced in response to elevated serum calcium, and decreases osteoclastic activity while promoting osteoblast activity.

Hypothyroidism is a decreased level of thyroid hormone in the body. It can be primary (an issue with the thyroid gland) or secondary (an issue with the release of TSH from the anterior pituitary). The most common cause of hypothyroidism in the western world is autoimmune, specifically two diseases, Hashimoto's thyroiditis and atrophic thyroiditis. In the developing world, low iodine intake is the most common cause and should be considered in malnourished patients.

Other causes of hypothyroidism are listed below:

  • Primary
  • Insufficient functioning tissue (e.g. Congenital, autoimmune-hashimoto’s, surgical removal, other iatrogenic causes)
  • Impaired thyroid hormone synthesis (iodine deficiency, congenital enzymatic defect)
  • Secondary
  • Insufficient secretion of TRH or TSH - hypothalamic disorders such as tumour, irradiation or inflammation, hypopituitarism caused by mass lesions or other damage to the pituitary
  • Thyroid hormone resistance syndrome

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