BLOG: When thyroid labs do not add up
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I was recently in our endocrine clinic, reviewing labs in between patients while also supervising three fellows. Once I saw this thyroid lab report (Figure), I paused, and then, I smiled.
I turned to the fellows. I asked them to take the challenge of a quick question, what we call a “spot diagnosis” in clinical medicine. Medical education is tough, but it can be entertaining, which is why I like academic medicine and teaching.
Medicine is full of tough moments. One such moment, for example, is providing emergency care to a very sick patient in the ED or in the hospital whose life depends on a correct diagnosis and management. Another stressful situation is encountering a non-acutely ill patient in the clinic who may not be satisfied with the doctor’s care, and that doctor may encounter disrespect that will make the rest of his or her day miserable and sad. Then there are the day-to-day stresses: dealing with insurance companies, stringent system regulations, seemingly never-ending paperwork.
So, I try my best to instill some fun in medical education when teaching medical students or trainees. I like to ask trivia questions with symbolic, $2 awards; what I call $2 questions. I developed this teaching tool over a decade ago; every now and then, I ask these $2 questions to fellows, residents and students in clinic and hospital.
Looking at the thyroid lab results, I already know the patient's case from seeing her unique last name. So, I asked the fellows: How do you interpret these thyroid labs?
The fellows asked for some clues. One question: What is the clinical picture of the patient — hyperthyroid, hypothyroid or asymptomatic?
“Asymptomatic and, in fact, euthyroid,” I answered. After providing the clue, I noted that this was now a $1 question.
The main two thyroid labs that people are familiar with are thyroid-stimulating hormone and free thyroxine. TSH, or thyrotropin, is a hormone produced by the pituitary gland, an olive-shaped structure housed at the base of the brain in a chamber called the sella turcica. The pituitary secretes major regulating (stimulating) hormones, such as TSH, luteinizing hormone (LH) and adrenocorticotropic hormone (ACTH), that control the function of the target glands: thyroid, the gonads and the adrenal glands, respectively. The target glands then produce corresponding target hormones: thyroid hormones (triiodothyronine and T4); testosterone/estrogen; cortisol, in the same order. What is so intriguing about endocrine labs is that the pituitary hormones are opposite to the target hormones; this is called negative feedback regulation. When we see low free T4 and high TSH, this suggests hypothyroidism; however, this only applies to disorders of target glands, the so-called primary endocrine dysfunction, such as primary hypothyroidism. It does not suggest secondary disorders, such as diseases of the pituitary.
The labs shown in the figure are thus not adding up. Since TSH is this high, free T4 should be low. Instead, free T4 is normal. In this case, and in similar cases where the labs do not add up, one would think of certain medical conditions or of lab artifacts, or problems with lab assays.
After I provided the clue, the fellows guessed the correct answer.
The patient had thyroid cancer and underwent total thyroidectomy. These labs were ordered by us as a routine lab to monitor her thyroid hormone dose. But the labs were completed by the patient just 3 days after receiving two consecutive injections of Thyrogen (thyrotropin alfa, Genzyme), a synthetic TSH analogue. In this case, the TSH level in the blood will be high because of the exogenous TSH.
I called the patient.
“When did you take the Thyrogen?” I asked.
The patient laughed and apologized for undergoing lab work at the wrong time.
“I am sorry that I did the lab soon after Thyrogen,” the patient answered.
The fellow who correctly answered first won the $1 award.
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