The invention includes a nucleic acid having a sequence at least 98% homologous to SEQ ID NO: 1, which encodes the .alpha. subunit of canine thyroid stimulating hormone (TSH). The invention also includes a nucleic acid having a sequence at least 98% homologous to SEQ ID NO: 2, which encodes the .beta. subunit of canine TSH. The invention also includes a method of producing an recombinant canine thyroid stimulating hormone (rcTSH) subunit by expressing a nucleic acid having a sequence of SEQ ID NO: 1 and a nucleic acid having a sequence of SEQ ID NO: 2 in a transgenic insect cell modified to sialylate proteins and producing a sialylated rcTSH subunit. The insect cell may be a lepidopteran cell. The rcTSH may be used for diagnosis and treatment. It may be used to diagnose canine hypothyroidism.