Thyroid lab tests explained in under 150 words

Q. I have been trying to figure out the two basic thyroid lab tests, TSH and T4. If you have a high TSH and a low T4 does that mean that the pituitary gland is going crazy to reach homeostasis but the thyroid is not responding? And inversely, if the T4 is high and the TSH is low does that mean for some reason the thyroid is working overtime due to a disease like Graves disease, and the pituitary is trying to compensate by not producing TSH?

A. Yes! That’s exactly right. When the two (TSH and T4) are opposite of each other – high T4/low TSH or low T4/high TSH – that means that the problem is intrinsic to the thyroid gland (Graves disease or Hashimoto thyroiditis, for example) and the pituitary is trying to control the thyroid by producing more or less TSH. Those are the most common types of thyroid disease – those that are intrinsic, or primary to the thyroid gland itself.

On the other hand, if both TSH and T4 are either low or high – high T4/high TSH or low T4/low TSH – that means that the process is being driven by TSH. Either there’s a pituitary adenoma making a ton of TSH, or the pituitary is not working well for whatever reason (it’s been radiated, or has undergone necrosis) and it’s not making enough TSH.

Image credit: akay (http://www.flickr.com/photos/akay/245002004/), under cc license.

What does normal parathyroid tissue look like?

parathyroid

Might as well admit it: the histology of the parathyroid glands is highly forgettable. I’m not sure why this is – perhaps because we don’t talk about it very often, perhaps because it’s so banal – but I do know that it’s one of those things that needs a lot of repetition to become permanent.

The parathyroid glands are four dinky little things (3-4 mm, about 35 mg each) usually located on the posterior surface of the thyroid gland. They exist in two pairs; the upper pair is derived from the fourth branchial cleft and descends with the thyroid gland, and the lower pair is derived from the third branchial cleft and descends with the thymus.  Their main function is the regulation of serum calcium levels. They do this by secreting a hormone called parathormone (or PTH) when serum calcium levels go down. PTH does all kinds of things (it activates osteoclasts to chew up bone, increases renal reaborption of calcium, increases renal conversion of vitamin D to its active form, and increases calcium absorption from the gut) but the bottom line is that it raises the serum calcium.

It’s the histology that’s like a blank spot in most medical students’ (and physicians’) minds. The parathyroid is composed of two types of cells: chief cells (small, round, bland cells) and oxyphil cells (large cells with abundant eosinophilic cytoplasm). The chief cells are the secretors of PTH, and they make up the bulk of the cellularity of the parathyroid. Scattered throughout are small islands of oxyphil cells (you can see one at 12 o’clock in the image above). There is a varying proportion of fat, too, that increases with age (like every other part of the body, it seems).

That’s it! Now go memorize it! Maybe this is the final repetition of parathyroid histology that will stick in your brain forever.

Lymphocytic thyroiditis

lymphocytic-thyroiditis

The final member of the thyroiditis quartet is lymphocytic thyroiditis (also called silent thyroiditis). This type of thyroiditis is characterized histologically by – you guessed it – a ton of lymphocytes (as in the image above). (more…)

Hashimoto thyroiditis

hashimoto thyroiditis

We’ve been discussing thyroiditis lately (see posts from 4/27/09 and 4/28/09). There are four kinds of thyroiditis: Hashimoto, subacute granulomatous, lymphocytic, and fibrosing. (more…)

Vascular invasion

There are four types of thyroid carcinoma: papillary, follicular, medullary, and anaplastic carcinoma. (more…)