Thyroid Function Tests

00:01 What are my key thyroid function tests? The most sensitive is TSH.

00:07 For example, if you find a patient with Hashimoto and there was decreased levels of T3, T4, what are you going to measure? TSH and you find that to be elevated.

00:17 What if your patient had secondary hypothyroidism? That means that the anterior pituitary is not functioning properly.

00:22 Your TSH levels will be decreased, thus T3, T4.

00:24 The TSH levels allow you to ask if a change in thyroid hormone is being driven by the pituitary.

00:32 For example, if T4 is low and TSH also low then where’s my underlying issue? Not in the thyroid, it is in your anterior pituitary.

00:44 Free T4 – directly measures the free hormone.

00:48 Free T4.

00:49 Very sensitive, bypasses effects of disease and drugs on thyroid binding globulin levels.

00:56 Free T4.

00:57 We’ll talk more about TBG.

01:00 Thyroid binding globulin would be the binding protein that’s being synthesized and released by the liver, influenced by the hormone estrogen that we’ll talk about later in pregnancy.

01:12 Total T4.

01:13 Total.

01:14 What does total mean to you? It’s a concept.

01:17 Free plus bound.

01:18 Free plus bound is total.

01:20 When you have total, which one of those components makes up majority of your total? Not the free.

01:27 It’s the bound.

01:28 Clear? So, total means free and bound.

01:32 Always keep that in mind.

01:33 That will make your life so much easier later when we talk about pregnancy.

01:37 So, total T4 greater than 99% protein bound… What’s the name of that protein? Thyroid binding globulin.

01:45 Coming from where? From the liver.

01:47 Influenced by whom? Estrogen.

01:48 I’ve done that plenty.

01:50 Need to take into account TBG status and thus, helps in diagnosis of hyperthyroidism, hypothyroidism only under conditions of normal TBG levels.

02:01 So, you need to make sure that you rule out properly where or not there is increased levels of TBG.

02:09 And that will be important for you.

02:11 So, what is TBG? Thyroid binding globulin binds T4 and T3, comes from the liver.

02:16 It is then influenced by estrogen.

02:17 Now things that you’re paying attention to what the type of patient who increases the levels of TBG include pregnancy or contraceptive pills with, that may contain estrogen.

02:30 And decreased by androgens, glucocorticoids, severe illness, malnutrition and protein-losing states.

02:39 Those are decreased in TBG.

02:41 You want to make sure that you know about those patients and you want to know about those patients in which you find increased TBG, those indications where you have a patient and an increased estrogen status.