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Review of Thyroid Physiology
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Slides Thyroid Disease.pdf
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Download Lecture Overview
00:01 Today we’re gonna talk about thyroid disorders. 00:03 Let’s start by revising some of the thyroid physiology. 00:08 Hypothalamic thyroid releasing hormone triggers the pulsatile release of TSH on the anterior pituitary. 00:15 TSH being thyroid stimulating hormone. 00:18 This activates thyroid cell growth, it has a role in effecting iodide metabolism, and then finally also causes the synthesis of thyroid hormone within the thyroid gland which mainly manifest as T4 and T3. 00:34 T4 and T3 once produced, then feedback negatively on the hypothalamus to reduce TSH. 00:41 The ratio of T4 to T3 secretion is nearly 20:1 Most T3, in fact 80% of it requires deiodination in the liver and kidney to become T3 Only a tiny amount of free T4 and T3 is biologically active, the rest being bound to circulating proteins. 01:04 Thyrotropin or TSH is the best single test for screening and function of the thyroid. 01:10 If the TSH is abnormal, additional evaluation of the thyroid function should be considered to determine the extent of the dysfunction. 01:17 Measure T4 when thyroid stimulating hormone is elevated and measure both T4 and T3 when thyroid stimulating hormone is suppressed. 01:28 TSH is very sensitive for detecting thyroid dysfunction. 01:32 Check the T4 level with the TSH level to evaluate for central hypothyroidism. 01:38 The TSH level may reflect hypofunction when TSH is high. 01:44 Hyperfunction when TSH is low or be within the normal range which is usually in the 0.5-5 milliunit per liter range. 01:54 There are three factors that alter the range of normal TSH levels. 01:58 Pregnancy, aging and pituitary dysfunction. 02:05 Checking a serum T3 is necessary if the patient has a suppressed TSH level because some patients with thyrotoxicosis, only T3 may be preferentially secreted over T4 and this is a condition known as T3 toxicosis. 02:21 This is also very very rare. 02:24 In patients with an elevated TSH level which indicates hypothyroidism, T3 will be within the normal range even with significant disease.
About the Lecture
The lecture Review of Thyroid Physiology by Michael Lazarus, MD is from the course Thyroid Disorders.
Included Quiz Questions
Which of the following is true about thyroid physiology?
- T3 and T4 cause negative feedback by inhibiting the hypothalamic-pituitary axis.
- TSH is released continuously to synthesize T3 and T4.
- The ratio of T3 to T4 production is approximately 20:1.
- TRH directly increases iodine metabolism in the production of T3 and T4.
- T4 is biologically active and is the de-iodinated form of thyroid hormone.
When should hyperthyroidism be suspected based on laboratory tests?
- When the TSH level is below the normal range, with elevated T3 and/or T4 levels.
- When the TSH level is above the normal range, with low T3 and T4 levels.
- When the TSH level is above the normal range, with normal T3 and T4 levels.
- When the TSH level is below the normal range, with low T3 and T4 levels.
- When the TSH level is above the normal range, with elevated T3 and/or T4 levels.
Author of lecture Review of Thyroid Physiology
Michael Lazarus, MD
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