How I manage subclinical hypothyroidism during fertility treatment and the first trimester of pregnancy
There is a simpler way to adjust thyroid hormone dosage
Optimal management of subclinical hypothyroidism ensures treatment success and a smooth first trimester. A dose increase is needed as soon as the pregnancy is established, because fetal thyroid gland is not functional until the second trimester.
Ms. Boyd received a diagnosis of subclinical hypothyroidism six weeks ago. She took 50-mcg Synthroid daily since then. Her blood test today showed TSH (Thyroid Stimulation Hormone) is 4.1 mIU/L. I asked her to take two tablets on Sundays and return in six weeks to repeat the TSH test. Dr. Torres, one of our fellows in the clinic, asks why I increased Ms. Boyd’s dose this way rather than sending a 75-mcg Synthroid prescription, the next higher dose.
Like most fertility specialists, my TSH goal for Ms. Boyd is 2.5 mIU/L. So, Ms. Boyd needs more thyroid hormone. The way I did it provides more flexibility for future dose adjustment (if needed), avoids a new prescription, and wastes the old medication. Ms. Boyd will not experience a surge of thyroid hormone on Sundays when she takes the double dose, because > 90% of the circulating thyroid hormone is bound by albumin and not biologically active.
Taking two tablets on Sundays and one tablet the rest of week, Ms. Boyd increases her dose by 14 % (+ 1/7). If a 7% (+ 1/14) increase is desired, she can take two tablets every other Sundays. Similarly, she can achieve a 28% increase by taking two tablets on Saturdays and Sundays (+ 1/7, + 1/7). The above method works both ways: she can skip Sundays and take one tablet the rest of the week to reduce the dose by 14% (- 1/7).
When I prescribe Synthroid to patients for the first time, I remind patients to increase their dose by 1/3 as soon as a pregnancy is confirmed. For those taking one tablet a day, taking two tablets every three days is easy.
For those already taking two tablets on certain days of the week (such as Ms. Boyd), I calculate the new daily dosage as follow: 1) count the number of tablets in a week, 2) multiply by 1.33 and round it up to the next higher number, 3) this number is divided by 7, the integer quotient is the number of tablets to be taken daily, and the remainder is the number of days one more tablet is needed.
Using Ms. Boyd as an example, and assuming the optimal dose was 8 tablets per week, below is how to calculate the new dose during the first trimester: 1) total number of tablets is 8, 2) 8 x 1.33 = 10.64, round it up to 11, and 3) 11 divided by 7, the quotient is 1, and the remainder is 4. So, once Ms. Boyd is pregnant, her new dose during the first trimester is: 2 tablets daily 4 days of the week, and 1 tablet the rest of the week.